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.”

https://en.wikipedia.org/wiki/Solder#Lead-free_solder

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.

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.

Temperature

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.

Edges

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.

Devitrification

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. 

Annealing

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.

Testing

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.

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.

There is a much better cleaner, especially for removing kiln wash and investment mould material.  It is a neutralised acid - tri-sodium citrate.  It is often sold in the dihydrate form.  It is used in the food industry and so is widely available. Glass can be left in the 6% solution for days without being etched.  The unwanted material is chelated from the surface during this soak.  Occasional agitation may speed the effect by removing the loose material on top.

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.

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.

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.

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.

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.

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.

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.