Wednesday, 22 February 2017

Flip and Fire

"Flip and Fire" is a term was devised by Brian Blanthorn to describe a process to achieve crisp details in the final piece.

The process takes advantage of two things - heat and weight. The glass on the shelf side moves less than the top as it is not quite so hot, and the weight of the glass above keeps the lines the way they were cut. The glass on the top of the piece begins to move first and fill the gaps that are left between the pieces.

This piece has been assembled with the final upper surface on the shelf and the base sheet placed on top.

The simplest method to achieve straight lines is to fire the piece with the final surface down to the shelf. After fusing, turn over and clean any surface contamination, usually by sandblasting. Wash and polish dry. Then fire the new surface to a fire polish temperature.

The same piece fire polished after cleaning the fused glass.

This technique works best on pieces that are of one uniform thickness.

There are other factors at play in obtaining crisp lines. 

Fusing straight lines

In addition to the "flip and fire" approach, there are a number of other factors that contribute to sharp, crisp lines in a piece made with strips of glass laid on edge and fused.

Smooth glass will fuse straighter than strips of textured glass. The individual strips fit closer together, leaving less room for lines to wander and create a wavy appearance.

The quality of the cut of the strip is important. Straight strips with right angle edges and no flares make for crisper lines.

The thinner the strips, the less opportunity for movement in the fusing when they are placed on edge. Ideally, the strips should be 6mm wide. This is the thickness that glass tends to take up when full fused. The greater the width beyond 6mm, the less likely the lines will be straight.

The viscosity of the glass affects the crispness of the lines. A glass that is less viscous will tend to be more wavy than a more viscous glass. E.g., black glass, a less viscous glass than white, will tend toward waviness more than the white. This is not a variation between manufacturers; it is a variation within a compatible range of glasses.

The firing surface will have an effect. Firing directly on a kiln washed shelf will give crisper lines than firing on fibre paper of whatever thickness.

Damming the composition before firing will produce straighter lines. The dam holds the strips in place during the heat up and restricts any flow that would be caused by strips thicker than 6mm.

Solder for Zinc

A number of people seem to have difficulty soldering zinc around their projects.  This is because zinc transmits the heat quickly – more quickly than the tin/lead solder – requiring more heat to be put into the process.  There is a solder that can make this process easier as it is designed for soldering zinc.

“Galvanite is a lead-free galvanizing solder formulation designed specifically for high quality repairs to galvanized steel surfaces. Simple, effective and easy to use, in both manufacturing and field applications. It metallurgically bonds to the steel, for a seamless protective barrier.”

It composition is 50% tin, 49% zinc and 1% copper.  It becomes solid at 200C and liquid at 300C.  This makes it a high temperature solder for stained glass purposes, but will give a firm attachment between the zinc and the solder or lead came it surrounds.  The high temperature aspect means you need to keep the iron on the zinc rather than the more easily soldered metals or the glass.

Wednesday, 15 February 2017

Single Layer Slumping

Almost all glass can be slumped as a single layer, whether produced for kiln working or not.  A few are extra sensitive at even slumping temperature and change character at around 630C-650°C, but all others can be slumped.  This posts concentrates on slumping of single layers of non-fusing compatible glass, but most of these elements can be applied to fusing compatible glass too.

The things you need to take care about are:
  • Temperature
  • Soak Times
  • Edges
  • Devitrification
  • Annealing
  • Testing
It certainly is possible to slump single layers. The resulting glass will be slightly less robust than two or more layers of glass, but simply because it is thinner.

The temperature that you use needs to be high enough to allow the glass to take the shape of the mould, but low enough that the glass does not distort or stretch and thin.  This is a balance that you can achieve through observation of the firing. 

It most often is best to use the lowest practical forming temperature that you can.  Practicality here is about how long you want to wait for the glass to conform to the mould.  It is possible to take the glass to about 580°C and soak for multiple hours, but not very practical.  It does depend on the glass as to the temperature to be used for the slump.  There are two sources here that can help: the slump point test  and this table of glass characteristics

Soak times
A practical soak time will be 30 – 90 minutes, which will avoid marking the underside of the glass.  This means that the temperature will need to be lower than the softening (or slump) point of the glass. Your slump point test will tell you the temperature at which the glass begins to deform.  That is the best temperature to use.  If it is taking too long, advance the temperature by about 10°C.  If you used the table of glass characteristics to find a softening point, reduce that temperature by about 30°C as a starting point.

The temperature that you will choose to use is not high enough to allow the edges to change as they would in a fuse.   This means that you need to have the edges exactly as you want them in the finished project.  This will require cold working by hand or machine.  Neither will take a long time, but require the correct tools. This post gives you the comparison of fused and cold working methods.

While most glass can be slumped you need to be careful with opalescent glass, as it can devitrify easily.  Most wispy glasses are fine, but the more opalescent wisps they have, the more difficult there may be.  Streaky and single colour glasses are usually fine. 

Another element in slumping glass not formulated for kiln working is the annealing of the glass after the slumping.  The annealing temperature can be estimated as 40C below a low temperature slump of a 280mm span of glass. The slump point test mentioned earlier will help determine the annealing point. You need to soak for a time - maybe 30 minutes - at the estimated annealing temperature and then cool slowly in case you have miscalculated on the annealing temperature.  In any case, a long slow anneal cool will pay dividends in a more robust glass.

You will find some manufacturers’ glasses are less adaptable to kiln forming than others.  So, it is best to run tests on the glass before committing to larger projects.

Remember TADSET - temperature, annealing, devitrification, soak, edges, test.

Wednesday, 8 February 2017

Vinegar for Cleaning

Cleaning glass with acids causes corrosion of the surface of the glass.

So many people mention using vinegar to help clean the ground edges. I can't resist commenting. Vinegar is acidic. Glass is alkaline. Leave the glass in the vinegar too long and it will affect the surface of the glass.

Sometimes it dulls. Sometimes it corrodes to give a mild iridised appearance. The acid removes the alkaline materials – potash, lime, etc. – leaving a pitted surface at the microscopic level.  Left long enough – hours rather than days – the surface will begin to appear dull due to the pittiing. It is at this stage that it is easy to introduce contaminants which may later form nucleation sites for devitrification.

If you must use vinegar, rinse with it. Do not soak your glass in a vinegar solution.

Alkaline cleaners

Two alkaline substances that are used to clean glass are baking soda and ammonia.  Both are effective cleaners and do not have a reaction with the glass as they both are alkaline. The glass can be left to soak for a brief time in a solution of these chemicals, although I would not be happy with an open bath of ammonia.

But the effective part of what people are doing to clean the edges is the scrubbing. Scrubbing the glass powder out of the pits left by the grinder is what really works.  When leaving the glass in a bath of even plain water, you are giving the powdered glass the opportunity to settle into these pits.  Once settled into the pits, the powdered glass can become like cement to remove.

Mechanical cleaning

You could have a much better effect if you scrubbed under clean water before placing in a bath of water with grinder lubricant.  This material promotes a gel like glass residue. This gel prevents the glass becoming caked like cement.

A final scrub to thoroughly clean before assembly is a good idea. Each piece should be polished dry with lint free cloths or uncoloured absorbent paper.  If any particles cloth or paper are left behind, they will burn away long before devitrification can begin to form.

Of course, the best solution is to grind with 400 or 600 grit.  This is fine enough that there is not enough powder left to promote devitrification.

Wednesday, 1 February 2017

Devitrification on Ground Edges

The first element in preventing devitrification is cleaning.  Making sure all the edges of the glass are clean will help.  OK, you have cleaned the edges well after grinding. You still get detrification, so you want to know

Why do ground edges get devitrification? 

To answer this question, you need to think about how glass behaves in the kiln. As it heats up the glass expands, pushing the cut edges into the separator on the shelf. The pits caused by the grinding have not yet become fire polished.

When the glass retreats on cooling the pits in the edges of the glass, although very small, pick up some of the separator. These small particles act as the nucleation points for the crystallisation of the glass which is generally called devitrification.

The glass of a single 3mm layer retreats further on a single piece than on a 6mm piece. This rolls the devitrified glass upward onto the upper edge of the piece.

Prevention of devitrification of the ground edge is to have the pits in the glass edge finer than the particles of the separator. This is more than just washing the glass immediately after grinding to remove the glass powder from the grinding scratches.  Yes, this will reduce the chance for devitrification, but not totally prevent it.  As noted above, the pits in the glass will pick up particles of separator on expansion, giving nucleation points for the devitrification.

Further coldworking beyond the initial grinding is required to reduce the devitrification possibilities.  This involves using finer grinding bits or smoothing by hand with finer grits.  This does not have to take long, as the shape has been achieved by the grinder.

The logic of prevention is to have the glass edge smoother than the particle size of the separator, so the finer and smoother the separator, the smoother the surface of the glass edge must be.  

But my devitrified edge was on top of other glass

The follow-on question is about why devitrification occurs on ground edges that are not near the kiln shelf.  There are two elements to consider.

It is claimed that the fumes of the binder burning off can settle in the pits of the ground glass, providing those nucleation points for the glass crystalisation. The suggested solution is to vent the kiln to about 400C to allow the combustion fumes out of the kiln rather than keeping them inside the kiln.

The second and more certain element is that the grinding creates microscopic pits and fractures in the glass where the powder from grinding settles.  Almost no amount of cleaning will completely remove this residue from the tiny pits and fractures resulting from grinding. 

There are at least two solutions to this cleaning problem. Don't grind unless absolutely necessary - groze instead.  The second is to lightly cover any ground edges with clear powder frit.  You could of course consider ultrasonic cleaning or power washing, either with a dishwasher, or outdoor power washer.  Both these seem to be so completely out of proportion to the problem, that I have never used them.

Tuesday, 24 January 2017

Mandrels for Screen melts

In creating screen melts, the steel or other support left in the project can leave such a degree of stress that the piece will began to fracture over time.  The use of thin stainless steel rods as used in mandrels for bead making is an alternative, as they can be pulled out.

The separator used on the mandrel can be bead release.  If you have it that will work very well. This illustration shows a bead maker coating a mandrel from a bottle of mixed bead release.

If you do not have bead release on hand, you can use kiln wash.  To give the thick coating required to easily pull the steel out you need to mix the kiln wash differently. 

The normal mix of kiln wash would be 5 parts water to one of powdered kiln wash.  As you want this to be thicker so it will stay on the mandrel, you can mix it in a 3:1 ratio.  This will be sufficiently thick to keep it running off the mandrel and be able to extract it after kiln forming.

Mandrels prepared for bead making.  In coating them for a melt, you need to have the whole length coated.

To avoid the mess of pouring the wash over the mandrel, you can fill a stringer tube with the mixture and dip the mandrel into it. You can place the end of the mandrel into a polystyrene insulation block or a bit of clay to let it dry as done by bead makers.

Once dry, you can arrange these coated mandrels in any shape of grid you choose.  Lay them across your supports whether fibre board or brick with about 25mm on the support at each end.  Lay all of one direction down first and the follow with the second, or more layers.  Place you glass on top of the grid created and fire.

Wednesday, 18 January 2017

Assessing Pre-programmed Schedules

Many kiln manufacturers are shipping their kilns with a set of programs already entered and saved into the controllers. 

You might think that all these pre-set schedules would all be the same, as the range of glass to be considered is relatively small. Yet, the range of schedules for the same glass varies from one manufacturer to another.  Yes, you may respond, but every kiln is different.  Well, I’d say, the variation is within a product line as much as between kiln manufacturers.

Assessing the installed schedules

What this means is that you need to assess the schedules that come with your kiln, rather than simply accepting what has been placed there.  There are a few things that can be looked at to assess whether you wish to rely on these pre-set schedules or not.

Differences between fast and slow fuses. 
  • ·         What are the initial rates of advance, are they different?
  • ·         Where is the bubble squeeze, is there one?
  • ·         Are there different rates of advance from bubble squeeze to top temperature?
  • ·         If you can compare their larger and smaller kilns, is there a difference in schedules?

Differences between tack and full fuses
  • ·         Are the top temperatures different for tack and fuse?
  • ·         Is there more than one tack fuse temperature to allow for various levels of tack from lamination to fully rounded?
  • ·         Is there a difference in soak times at the target temperatures?

Differences in slump temperatures
  • ·         Are there low and high temperature slumps?
  • ·         Is there a difference in temperature or time between various slumps?
  • ·         Is there any allowance for span or size of mould?
  • ·         Does depth of the mould make any difference to the schedule?
  • ·         Is a difference for the depth of the mould offered?

Differences for different manufacturers’ glasses
  • ·         Are there different schedules for Spectrum, Wissmach, Bullseye, etc. fusing glasses?
  • ·         Are float glass schedules any different for rates, soak times, annealing points?

Printed schedules
  • ·         Are the schedules printed in the kiln handbook or manual?
  • ·         Are you given clear instruction on when to alter the programs?
  • ·         Are you given clear instruction on how to alter the programs?

The more “no” answers you get to these questions, the less you can rely on the installed schedules.

Wednesday, 11 January 2017

Holding the Cutting Head

Many people hold their cutting head steady with a finger during the scoring process.  This is not necessary.

The axel of the cutting wheel is slightly forward of the centre line of the cutter.  In addition, the cutter is held slightly angled back toward the operator to be able to see the wheel and the cartoon (or marker) line.  Both of these act to ensure the wheel follows the movement of the arm or body in a forward motion. 

In cycling, the distance between the angle of the shaft of the cutter and the axel is called the “trail”.  The greater the amount of trail, the easier it is to keep the bicycle following a straight line. The same applies to the cutter. This trail is created by the extension of the angle of the cutter to the glass.  The axel of the wheel is behind that line. The cutting head has a sharper angle at the back than the front to accommodate this angle backwards. The resultant forward force is in front of the axel and so leads the wheel to follow the direction of the cutter without any need for stabilisation.

There is no need to have your finger on the cutting head. It swivels for a reason. It will follow the direction you are pushing without any angle, so there is a clean score.  If you attempt to stabilise the cutter head you risk the wheel running at a slight angle to the direction of the score.  I talk about this as a skidding score. The result of this is to give a score with forces directed not only straight down but sideways too.   This gives the glass many more ways to break.  And not always along the line you want.

Also when you want to score a tight curve, the slight movement of the head allows the curve to be slightly smoothed again without any skidding.  This means there will be fewer pressure lines sideways to the score line.

Manufacturers have put the play into the cutter heads for a reason.  The above attempts to explain it.  The manufacturers would not include a feature that costs time and effort, as well as cost if it had no purpose.  It seems perverse of us to try to run counter to that by holding the head or even fixing it solid, so it is unable to pivot at all.

Wednesday, 4 January 2017

Encapsulation of Fused Glass Panels

You can encapsulate both leaded and fused glass panels into double glazed units.

Leaded glass panels have the outer came built in “Y” shaped came.  The tail of the “Y” is held between the spacer bars at the edge of the double glazed unit. You normally need to leave 25mm space on each side.  It is calculated as the reduction from the glazing size of the opening.  This is required to accommodate the width of the heart of the came, the spacer bars and sealant of the double glazing assembly.

The same sizing guide is required for fused glass panels.  Usually the “Y” came is designed for 3 mm glass, as it has a 5mm high heart.  However, the “Y” is broad enough that the leaves can be opened to accept the thicker 6mm fused glass.  It is also possible to grind a bevel on the back of the fused glass to make it easier to slip into the came.

It is best to find the person or company which will be making the double glazing unit before starting.  Discussion with them at the start will enable you to determine how much allowance is needed for the spacer bars and the sealant. This will assure you in setting the dimensions for the panel you will make.

Wednesday, 28 December 2016

Making Your Own Schedules

Starting out with your own schedules is a bit frightening as you don’t yet know the capabilities of your kiln and the problems that might occur. This note attempts to give you some pointers on how to go about making your own schedules.

Start with the glass manufacturer’s recommendations.  Picking something from the internet or a discussion list may seem easy, but you cannot assess the quality of the posted schedules.  Many odd practices have crept into the kiln forming community. The manufacturers know their glass, so you should start there. They are the quality control standards for kiln forming.  Modifications will of course be required for your particular practice as it develops.

Enter the manufacturer’s schedule for the project you are working on and then watch while firing.  Watching does not mean staring into the kiln.  This would damage your sight after a while. This watching consists of quick peeks into the kiln to see what is happening.  These peeks will be at above 580C.  It is only then that there is enough light in the kiln to see what is happening.  At first the peeks will be at possibly only 30 minute intervals.  But as you near the target temperature, you will need to peek at possibly 5 minute intervals. The progress of the glass forming will be much quicker, so to know when the right temperature has been achieved, frequent peeks will be needed.

This observation will let you know if the glass is achieving what you want. If it is not, you can change the schedule while firing.  E.g., advancing to the next step in the schedule, extending the soak time, changing the working temperature to a higher point.  Be sure to read your controller manual to ensure you know how to do these changes during the firing.

If you have achieved the look you want before the target temperature has been achieved, advance the schedule to the next segment or ramp.  Record this temperature, as the next time you fire this set up you will want to be 5°C -10°C lower than this time.  You are aiming to achieve your look with a 10 minute soak.  So, depending on temperature, rate of advance and your kin, this lower temperature with a 10 minute soak should achieve your desired look.  Record this schedule. You will need to observe the next firing just to be sure the temperature and time combination you choose works. 

If the desired look has not been achieved by your top temperature and soak, you can raise the temperature 5°C -10°C, even if you have to interrupt the firing to change the temperature.  The controller will recognise which ramp is required to complete the ramp to the new top temperature without going through all the segments of the schedule.  Even if it does not, you can advance to the ramp you need.  The effect of these changes will be minimal in relation to the full and uninterrupted schedule and can be relied upon to work well on future firings.  Record the new schedule for future use.

An alternative to the change of the top temperature, is to extend the soak when the temperature has not achieved the effect.  You will need to keep peeking until the sought for profile is achieved.  Record this new soak time and the results for future firings.

As you can see it is important to record schedules, layup and results every time you fire.  This enables you to compare results and learn.  A log provides a good reference when you want to reproduce something that was successful.  It also records what did not go well and can remind you of what to avoid.

This process of observation, amendment on the fly, and recording actions and results helps you to get to your ideal schedule much quicker than by putting a schedule in and coming back the next day to see what has happened.

Wednesday, 21 December 2016

Diurnal Firing Practices

It is most common for people to fire overnight so they can see their piece(s) the next morning.   This is a poor practice for novices.  Not simply a lazy one. It is a practice that leads to use of others’ programs and practices, rather than building on one’s own experience and practice. Others’ programs are used because they were successful for them.  They may not be successful for you. The number of failed projects that are discovered when the kiln is opened, show that it is often not possible to transfer another’s schedule to your project.

The ability to fire while you are absent is a great advantage to kiln forming practices.  The widespread use of the controller has brought many advantages to kiln formers, not least that they can get some sleep and have a social life. They no longer need to be beside their kiln all the time it is firing.  The controller has also made it possible to set the ramp rates and soaks without calculations.  And without having to set periodic alarms to remind us to check the kiln to see if it is advancing at the correct rate.

Before controllers it was necessary to sit beside kiln to watch what was happening and adjust the ramps and soaks to conform to what was planned.  It was also necessary to observe how the glass was behaving and adjust the power input accordingly. Now we can set the controller to give what we hope will be a good result.  We find out when we open the kiln in the morning whether it is right or not.

I am not advocating returning to the days before controllers.  I enjoy my sleep and social life too much for that.

I am advocating the use of a feature almost all controllers have.  The Delay function.  On most controllers, it is the first thing that comes up on the display.  We mostly ignore that and proceed to the first ramp.  We set the controller to fire immediately, so that it will be done overnight and we can look in the morning or when we come back from work.  That way all the waiting for the piece to be finished can be eliminated.  We can go to work or to sleep knowing that the firing will be done when we can get back to the kiln.

This practice leads us to miss the real learning process that is available by observing the process of the firing.  Observing the firing can tell you when your slump is done, when it is slipping to the side, when it breaks, when more time is needed, when more heat is needed – almost everything that people ask questions about their slump – or in other instances, the fuse or melt.

People ask what temperature they should use for the kind of tack fuse they want.  Many suggestions can be made.  Trial and error will eventually tell which is the right combination of rate, temperature and time for the result you want. Observation during the firing will tell you immediately when the temperature is high enough, or the soak is long enough.  As you peek into the kiln through the observation ports you can advance to the next ramp when you have achieved the look you want. 

You do have an observation port, don’t you?  It is one of the essential features to be included in a kiln. If you don’t have one, you can open the lid or door momentarily to observe the state of the glass in the midst of the firing.  You could make an observation port by drilling through the casing and insulation.  You then place fibre blanket or a formed piece of kiln brick in the hole when not in use for peeking into the kiln.  You will not change the performance of the kiln by doing this.  Of course, if you have side elements, this retrofitting of an observation port is risky.

“I need a life.  I have to work.  I have to sleep.  I can’t be around my kiln all the time.” 

The legitimate responses to the idea that you should be around to observe the work at critical temperatures are that “I need a life.  I have to work.  I have to sleep.  I can’t be around my kiln all the time.” This is where the Delay function comes to your aid. You can use the Delay function to make sure the firing is at the critical point for observation at a time that is convenient for you.  This way, you do not disrupt your normal life.  Your social life can continue and you can get some sleep too.

An example will help understanding how you can make use of the Delay function. 

If you have time before you go to work, you can set it so that the firing comes to the critical point about an hour before you have to leave for work.  Or if it is better for you, you can set it so that time for observation is after you get home from work, or after dinner, etc.  Even if you don’t have a day job, you can use the Delay function to make sure you will be able to give the kiln the attention it needs at a convenient time for you.

How do I do this?  It is a setting of the amount of time to elapse before the kiln starts to fire on the first ramp.  Most programmes have firing times for each ramp.  You select the cumulative times up to the end of the ramp for the observation to begin.

This does not need to be difficult

This sounds complicated?  Not really.  It is a bit of arithmetic, though.  Add the times for each ramp together to get the time the kiln will take to get to the observation temperature and soak. 

200C/hour to 630C for 30 mins   =3.15 hours plus the 0.5 hour soak.  (divide the target temperature by the ramp rate, in this example 630/200=3.15 hours or 3 hours and 9 minutes).   Don’t include the soak time in this calculation as that is the part of the observation that is or may be variable.

Assume it is 10:00pm and you want to look at it at 7:00am.  This is 9 hours. The kiln needs 3 hours and 9 minutes to get to the temperature you want to observe the slump.  Subtract 3 hours and 9 minutes from the 9 hours you have and this gives you 5 hours and 51 minutes to set in the Delay function. 

Some controllers do not allow hours and minutes, but require only minutes.  In this case, multiply the hours by 60 and add the minutes.  In the example, there are 300 minutes in 5 hours plus 51 minutes gives 351 minutes to be put into the delay function. 

In this example, this will have the kiln at 630C at 7:00 am. Ready for you to observe the progress of the slump.  When the slump is finished, you can advance to the next segment and head off to work, allowing the cool and annealing to proceed, and knowing the slump was successful.  The same applies to other times of the day.  You could, for example load the kiln and schedule the delay to be at the critical temperature for when you come home from work.

Setting the delay function for an exact tack fuse is a little more complicated.

If you are looking to get an exact tack fuse profile, the schedule will be a little more complicated.  Say you want a rounded tack fuse that you think will be achieved at 750C in 10 minutes.  The schedule might look something like:
Ramp 1: 200C/hour to 650C for 30 minutes =3.15+0.5 hours =3.65 hours
Ramp 2: 300C/hour to 750 for 10 minutes =0.33 hours (750-650/300) + 0.167 hours soak.

Adding these two ramps together gives you 3.95 hours.  Here you include the soak at bubble squeeze temperature in the first ramp, but not in the second, because that is what you are checking on. If it is 7:00am now and you won’t be back until 6:00, that is 11hours until you will be looking in at the progress of your piece.  So you subtract 3.95 from 11 and you set the Delay as 7 hours, 57 minutes (or as 477 minutes).  Then it will be ready for observation when you come home.

Won't I loose firing time by using the delay function?

You may feel that you are going to lose a firing by using the Delay function.  You often can peek into the kiln in the morning to see how things have turned out by using the overnight firing.  But there normally is still more cooling down time required. 

If you delay the top temperature until the morning to see what is happening, you still have the rest of the day for the kiln to cool and be ready for re-loading in the afternoon or evening.  During that time, you can be preparing the next firing.  So you have not lost any kiln time, but you have gained the knowledge that the firing is OK through any adjustments you made at the critical temperatures.

If you were to fire during the day to be able to open the kiln in the evening as your normal practice, you will lose one firing at the start of this kind of practice.  As you progress with the new practice, you will find that you do not lose the number of firings you are able to complete in a week.  Again, you are preparing the next kiln load while the kiln is cooling.

Yes, it does require doing things a little differently.  But essentially it moves the kiln preparation on by 12 hours.  That’s why I call it diurnal firing.  You are just changing by 12 hours your daily practice. You make things ready for the kiln to fire overnight, and prepare the new piece(s) during the day.  Or prepare the pieces in the evening to fire during the next day.  You still are preparing the next kiln load as the kiln cools off from the previous firing.

The extra planning effort is rewarded by more rapid learning

This little extra planning is rewarded by the ability to see what is happening in the kiln, so that you can adjust during the firing, rather than having to do a firing again, or in the worst case, completely re-make a piece after a disaster. 

You also learn much faster about the desired programmes required to get specific results.  Instead of doing multiple firings to find the exact temperature needed for the desired result, you can do it in only one or two firings.  This saves you lots of time, glass and electricity. 

Observation is really necessary for free drops – aperture drops, screen melts, pot melts, etc.  These require observation to get the desired results, as their progress is so variable from one firing set up to another. Using the Delay function will enable you to have the firing at the stage where observation is important when you are best able to be there to watch.

The alteration to your working practices to make use of the Delay function will be amply rewarded by the rapid learning that observation of the firing promotes.  This essential tool to aid in designing appropriate firing programs is too often ignored in teaching and using firing schedules.

Finally, it allows you to set up programs that are pre-set for your kiln and kind of work.  You will have learned the exact rates and temperatures and soaks to put into your programs.  These become your saved schedules that are tailored to your practice.

Wednesday, 14 December 2016

Edge Treatment in Cold Working

Frequently people who are grinding the edges of bowls, aperture drops and other vessels that need to have a smooth rim find that they are getting small chips of glass coming from the edge of the ground part of the glass.

There is a way to prevent theses unwanted chips  

The long established practice of glass workers has been to give the glass an arris at the end of each grinding stage before they change to a finer grit.  This small area of angled glass, allows the continued smoothing of the glass without creating such a sharp edge that the glass there is not strong enough to resist the grinding action.  

You will notice on a bowl or other rounded vessel, that the chips are almost always on the outside. The inside of the rim normally has an oblique angle to the rim, and the outside an acute angle.  The explanation is held in the angle.  As the rim is ground down, the outer acute angle becomes very thin as well as sharp.  At some point the glass is thinner than the grit used to grind the surface.  This causes little chips of glass to break off the edge.

By creating an oblique angle at the edge of the grinding surface, the glass will remain thicker than the grit being used to grind the glass.  If you feel you are taking off a lot of glass, it is advisable to check that the arris is still in place.  If not, give it a light grind to maintain the arris while using that grit. 

At the end of each stage of grinding, you need to add an arris for the next stage.  The reason for doing it with the coarser grit rather than the one you are about to proceed to, is that it maintains all the grinding at the same stage, enabling the whole piece to be finished to the same level of polish.

Wipe the surface dry and add marks with a paint marker.  Allow this to dry while you change grits.  The purpose of the marker is to assist you in determining when you have ground out all the previous marks, by the elimination of the paint.

Wednesday, 7 December 2016

Low Slumping Temperatures.

It is possible to have glass overhanging slumping moulds if you use low temperatures. The glass has the appearance of behaving differently at these low temperatures than at fusing temperatures.

Glass at low temperatures is affected largely by weight.  

At low temperatures it cannot quickly form exactly to the mould. It falls first in the middle.  Because the glass is not very plastic, the edges rise up from the mould at first, because the weight there is not great enough to allow the unsupported glass to bend.  The edges stay in line with the beginning of the bend in the middle. (apologies for the quick and dirty drawings)

If you have a mould with a rim, you will be able to observe this effect.  As the glass in the middle begins to slump, the glass at the edge rises from the rim.  This allows the central portion of the glass to settle and draw the excess glass into the mould.  

It only settles back to the rim with the heat work of the slump as the slumping soak continues.

It is useful to record the temperatures and times of these effects for different moulds.  It tells you when the slump begins, the intermediate stage(s) and completion of the slump.

Using the lowest practical slumping temperature gives the best results.
·         It allows glass with small overhangs of the mould to be successfully slumped. 
·         Low temperature reduces the mould marks on the back of the glass.
·         Fewer stretch marks are in evidence. 
·         Low slumping temperatures with long soaks reduce the uneven slump that is sometimes in evidence with deeper moulds.
·         Low temperatures allow different colours to heat more evenly.
·         Low temperatures reduce the thinning effect of a high temperature slump.

Wednesday, 30 November 2016

Large Shelves in Small Kilns

Sometimes you want to put a larger than usual piece in the kiln.  But your standard shelf is just too small.  There are some things you can do to allow the firing of the larger piece.

You can put a two part or multiple part shelf into the kiln. However, joining two or more small shelves to make a larger one will continue to show lines where they join.

You can put in a single larger shelf. If this is a mullite shelf, you need to be sure you can get your fingers into the gap to get enough purchase to lift the shelf back out.

You can put a large fibre or vermiculite board over the existing shelf and so extend the area.  You need to prepare these boards by firing and preparing them for firing with glass on top.

But, there may be problems with adding more shelf area. 

If you have a side firing kiln, the glass will be much closer to the elements, which will require baffles and certainly slower firing schedules.

The reason for the gap between the sides and the shelf are to allow air circulation underneath the shelf to get even heating and even cooling.  Restricting this air flow requires slower schedules. Some experts suggest cooling from one side – which this now will exhibit – requires annealing at half the rate of cooling from both sides, i.e., with air space along the sides of the shelf.  This means doubling the anneal soak, and a reduction by half of the annealing cool rate.

Wednesday, 23 November 2016

Kiln Washing Kiln Lids

It is frequently recommended that the bottom of the kiln should be kiln washed to prevent any spilled glass from sticking to the kiln brick.  You should remember that this is applicable to brick lined kilns.

This in itself is a little clue.  You do not need to kiln wash any insulation fibre in the kiln. If any glass were to stick to the fibre, it would come away easily.  In any case, most insulation fibre blanket will not stick to the glass.

The recommendation often goes on to advocate kiln washing the sides.  There is a caution that the side elements (if any) should not be kiln washed. The caution comes from the knowledge that water and electricity should not be mixed.  The kiln should not be on when applying kiln wash anyway.  If kiln wash is splashed onto the elements, it is simply a matter of letting the whole kiln dry naturally with the lid open before firing.

The extension to this series of recommendations is that the whole of the kiln should be kiln washed, including the lid.  This is not a good idea.  The wash on the lid will soon fail and drop dust and debris onto and into your work.  The glass should never touch the top of the kiln anyway.  If the elements are in contact with the glass, the glass will either stick to them or break.  You have to ensure you do not put glass nearer than about 20mm to the elements or lid. In any case, the glass will fall to the bottom of the kiln, not the top or sides – unless the kiln is not level.


The whole idea of kiln washing the interior of the kiln is suspect in some ways.  Anyone who has had glass drip off the shelf and onto the brick during an over-firing will know the glass eats into the brick through the kiln wash.  Kiln wash will only protect the brick at full fuse or less temperatures. But it is a good precaution to keep the pieces of frit that fall off the shelf from sticking to the brick. It does not do much more than that.

The application of kiln wash to the kiln creates another source of dust within the kiln.  Dust and general uncleanliness in the kiln is a main potential source of devitrification. Thus, the application of kiln wash should be the minimum necessary and does not need to go up the side beyond the elements or the lowest shelf height, whichever is less.

There is a strong argument to be made that laying a sheet of 0.5 mm fibre blanket on the floor of the kiln will provide better protection of the kiln than any amount of kiln wash.  It is less likely to fail, it is not a source of additional dust, it provides a better protection during any kiln runaway, and it is easily replaceable.

Wednesday, 16 November 2016

Thinking About Design

To think about design, you need a vocabulary to describe the object. This needs to be combined with a structure of principles. What follows is an outline to structure your thinking about design.  This is based on the writing of Burton Wasserman in Spark the Creative Flame, Making the Journey from Craft to Art, by Paul J Stankard, 2013.

First there is the vocabulary to structure the conversation about design. The elements of this are “… point, line, plane, texture, colour, pattern, density, interval, … space, … light, mass, and volume”

There are then principles of good design.  They relate to:
Unity – all the elements form a whole
Balance – note, not only symmetry
Rhythm – this can be repetition with or without variation
Emphasis – or contrast between a main element and the rest
Harmony – all the elements work together

These five principles of design together with the vocabulary of elements form the language of design and assist your critical thinking about expressing your design and realising it in the best way you can.  This thinking can be applied usefully to the critical appreciation of others’ works.

Saturday, 12 November 2016

Heavy Metals in Glass

Some concern has been expressed about the metals used in colouring glass.  This centres around the temperatures used in fusing and whether kiln workers may be of risk from these heavy metals vaporising.

First of all, let’s get some sense of perspective. This is from Greg Rawles, an acknowledged expert on the hazards of working with glass.

Understanding Exposure:

In reality, unless you are doing:
High-volume production work that exposes you to a health hazard all day long
You are exposing yourself to high levels of a health hazard for a brief time
You are working with a very toxic material
You are not working responsibly

You are not really at risk for an unacceptable exposure when working in a glass studio

Now, let’s think about how likely it is to have heavy metals vaporise at kiln forming temperatures. How stable would glass be if the metals that colour it vaporised when we fired it? the colour would vary with the heat and number of times we fired it.

Now, let’s think about how likely it is to have heavy metals vaporise at kiln forming temperatures. How stable would glass be if the metals that colour it vaporised when we fired it? the colour would vary with the heat and number of times we fired it.

Even if the metal were to evaporate, how much is in the glass. Apparently, Bullseye uses less than 3 pounds of cadmium for a pot of glass. We can tell from the sheet numbers that a pot of glass gives at least 2000 sheets of glass, so there is ca. 0.0015 lbs or .07 grams or less of metal in a sheet of 3mm glass. There is very little there to "vaporise", so even it were able to evaporate, it is in such small quantities as to be negligible, and the exposure so low as to be of extremely low risk. There is however, no risk in protecting yourself with dust masks. Just remember that the risks from vaporised heavy metals is much less than most of the other studio practices involving glass. If you need breathing protection for metals (and you may feel it is not worth the risk) then you need to be wearing a mask all the while you are doing glass work. It is about relative risk.

For complete information, the melting and boiling points of various metals relevant to glass colouring are given below.  The vaporisation will be somewhere above the melting point and toward the boiling point.  You will be able to see the relevant temperatures and take any precautions you feel are necessary.  Remember that the metals are not used in their pure forms, but as oxides.  These may have different melting and boiling temperatures.  In general, the oxides used in colouring glass have higher melting and boiling points than the pure metal.

Antimony -for whites
Melting point: 630C
Boiling point:  1635C

Antimony Oxide
Melting point:  380-930C
Boiling point:  1425C

Melting point: 321C
Boiling point:  767C

Cadmium sulphide - yellow
Melting point: 1650-1830C
Boiling point:  2838C

Melting point: 1907C
Boiling point:  2671C

Chromic Oxide – for emerald green
Melting point: 4415C
Boiling point:  7230C

Melting point: 1495C
Boiling point:  2927C

Cobalt Oxide- blue to violet
Melting point: 1900C

Melting point: 1084C
Boiling point:  2562

Copper Oxides - for blue, green, red
Melting point: 1232-1326C
Boiling point:  1800-2000C

Melting point: 1337C
Boiling point:  2970C

Gold Chloride - red
Melting point: 170-254C
Boiling point:  298C

Melting point: 1538C
Boiling point:  2862C

Iron Oxide – for greens and brown
Melting point: 1377-1539C
Boiling point:  3414C

Lead – for yellows
Melting point: 327C
Boiling point:  1749C

Melting point: 1246C
Boiling point:  2061C

Manganese Dioxide – purple and a clarifying agent
Melting point: 535-888C

Melting point: 1024C
Boiling point:  3074C

Melting point: 1455C
Boiling point:  2730C

Nickel Oxide – for violet
Melting point (II - for green): 1955C
Melting point (III - for black): 600C

Melting point: 221C
Boiling point:  685C

Selenium Oxide – for reds
Melting point: 118-340C
Boiling point:  350C

Melting point: 961C
Boiling point:  2162C

Melting point:  370C
Boiling point:   882C

Sodium Nitrate – a clarifying agent
Melting point: 308C
Boiling point:  380C

Melting point: 115C
Boiling point:  444C

Sulphur oxide - for yellow to amber
Melting point:  17C
Boiling point:   45C

Melting point: 231C
Boiling point:  2602C

Tin Oxides – for whites
Melting point:  1080-1630C
Boiling point:  1800-1900C

Melting point: 1132C
Boiling point:  4131C

Uranium oxide – for fluorescent yellow, green
Melting point:  1150-2765C
Boiling point:  1300C