Wednesday, 19 October 2016

Annealing Multiple Levels of Tack Fusing

A question was asked of me about schedules for tack fusing multiple pieces – three layers thick in places – as a single unit, then placing on a 6mm fused base and tack fusing.  Special interest was in how the different thicknesses and the tack fusing would affect the scheduling of the annealing.

My response – edited – was as follows.

This is going to be a long reply.  I have written a general guide to tack fusing that will be useful, but this response will try to be more specific to your project.

First, tack fusing of pointed things is more sensitive to annealing than rounded things.  For up to 3 layers of triangles, I would be thinking of annealing for at least 12mm (four layers). This means a 2hour soak at 482⁰C, followed by a cooling rate of 55⁰C for the first 55⁰C degrees and then 99⁰C for the next 55⁰C. After this 110⁰C degrees of cooling the rate can be as fast as 330⁰C/hr.  This will apply whether Bullseye or System 96 is involved.

Second, from the description I take it that a 6mm clear under a 3mm layer of two colours side by side is being fused as a base.  [This was confirmed], so you could fire at 200⁰C/hr to a bubble squeeze of 30mins and then 300⁰C/hr to top temperature.  Anneal at 482⁰C for 60-90mins and cool for first 55⁰C at 65⁰C/hr and the next 55⁰C at 150⁰C/hr, followed by 300⁰C/hr to room temperature.

The third stage is to combine them.  Think about how thick this is physically – ca.18mm.  Then think about the differences in thickness – 9mm.  My rule of thumb is to add the difference between thicknesses to the thickest part – in this case to 18 plus 9 equals 27mm.  This is the “scheduling thickness” for this variation with rounded elements.  As your piece has lots of triangles, you need more care.  It is an additional level of difficulty.  So I add another 3mm to my “scheduling thickness” to accommodate the angular aspect of the piece, making a total of 30mm for putting the two fused pieces together. 

This thickness leads me to propose a relatively complicated schedule.  I suggest 70⁰C/hr to 250⁰C, 100⁰C/hr to 540, 120⁰C/hr to 620 and then 150⁰C/hr to top temperature.  The top temperature will be lower than your normal tack fuse temperature because this is a much slower rate of advance than normal.  This in turn, means that you will want to be checking at intervals on the tack fuse progress from at least 720⁰C.

The annealing will be long and slow. About 5 hours at 482⁰C, 11⁰C/hr to 427⁰C, 20⁰C /hr to 370⁰C and 65⁰C /hr to 30⁰C. This will be a schedule of about 35+ hours.

The two sources mentioned earlier give the rationale for this kind of schedule.  Think about the considerations I have listed, and then decide whether I am being too cautious or not.  The principle remains - as you increase the risk factors, you
·         slow down rates of advance and cooling rates, and
·         extend soak times.

You should note that I have used Graham Stone’s Firing Schedules for Glass, the Kiln Companion and the Bullseye chart for Annealing Thick Slabs in preparing the proposed schedule, although you will not find this exact schedule in either of them.

Wednesday, 12 October 2016

Carved Fibre Moulds

The question of whether you can use carved moulds more than once will arise.

This refers to moulds made from refractory boards or materials.  Once fired, refractory boards and materials become more fragile as they have lost their binders.  If the carving is simple with lots of support, and the mould is kept supported in a container of some sort, rigidising is not essential.  The life of the mould may be short though.

To make a longer lasting mould, you can rigidise the refractory material using this method.  This can apply to board as well as blanket.  The process will make a much longer lasting mould that is light weight, and is not affected by rapid changes in temperature.

Do the fibre moulds need kiln wash?

This depends on both the nature of the material and whether hardened or not.  Refractory fibre boards – often called ceramic fibre – do not need kiln wash to separate the glass from the mould.  However, putting powdered kiln wash and smoothing it with a piece of glass or plaster’s float can give a less grainy finish.  If applied wet, the dried kiln wash can be gently sanded to give a very smooth surface.

Other refractory boards such as calcium silicate or vermiculite do need kiln wash to separate the glass from the mould.

Any refractory mould which has been hardened with colloidal silica will need to be coated with kiln wash to keep the glass from sticking.  The kiln wash needs to be re-applied each time the mould is used above tack fusing temperatures.  Otherwise it does not need renewal until or unless the kiln wash is chipped, scratched or in other ways damaged. 

Another popular separator is boron nitride.  It is sold under various brand names.  This must be applied each time the mould is used.

Wednesday, 5 October 2016

Rounded Bottom on Drapes

Sometimes drapes, such as the handkerchief drape over a cocktail shaker, finish with a rounded base.

The base is rounded because not enough time or heat was allowed to get it flat. The glass will benefit from a moderate, but steady advance in temperature all the way to the top temperature.  This rate will be around 100°C to 150°C per hour.  There is no need to speed the rate of advance at any time during the process of the drape.  Too rapid an increase in temperature may even give uneven drapes if there are differences in thickness or colour.  There is no need for a soak at the strain point on the way to the top temperature. Any thermal stress from the rate of advance - that some suggest may occur - will already have taken place by this temperature.

This slower rate of advance will mean that the glass will not dome so much on the drape.  It will have time both to conform to the top (which will become the bottom of the piece) of the mould support during the drape stage. 

You need to visualise what the glass is doing during the forming process. As the glass begins to drape, the glass on the support rises because it is not yet soft enough to stay flat on the supporting mould. It is only later at higher temperatures, that the glass on top of the support can conform to it.

If you watch the process – a really good practice - you will be able to tell when you have a good drape. And with this reduced rate of advance, you should have a flat bottom. And all of this may happen at a lower temperature than you expected.

Wednesday, 28 September 2016

Bubbles in Casting Mould Firings

There seems to be an increasing popularity for re-useable ceramic casting moulds.  One of the common problems with these moulds is bubbles.  

Frit size 
It rather depends on the sizes of the frit and cullet used as to how many and what kind of bubbles are created. The converse of expectations is what happens.  You get more small bubbles with powders and fine frits than with coarser frits.  The small bubbles rise and coalesce to form larger bubbles which rise more slowly as they have to push through a greater mass of material (just as in a liquid). Since glass is viscous, these little bubbles usually do not have time to push their way through the glass at fusing temperatures.  But at casting temperatures, there is less resistance from the glass, as it is less viscous, and so the bubbles can clump together and form the larger bubbles that burst through the surface.

Temperature range and rate of advance
The amount and kind of bubble also depends on the speed of the ramp and the bubble squeeze you give it. If you proceed rapidly to top temperature, you will have to go to a higher temperature, allowing the surface to become more plastic and be pushed out of the way by the expanding air that almost certainly is in the mix. A slow rise will allow all the glass to become the same temperature throughout without using a high top temperature, so reducing the risk of the bubbles pushing through the more viscous glass to the surface.

All these problems would be reduced by having a vent or sprue to allow the air out from the bottom. Almost all purpose made casting moulds have these things. Sometimes they are as thin as a few hairs (from somebody with long hair) to as thick as a toothpick. As you have to do some cold work on the results from these moulds anyway, a few little strands of glass should be no problem to clean up. If the manufacturers won't do it, it is possible to take your Dremel or similar drilling tool and with a fine drill bit and make these tiny holes in appropriate places.  

I do not understand why these casting moulds do not have tiny air vents at the bottom of the depressions. Yes, there would be a tiny pimple on the surface of the final piece, but this can be cleaned away easily. The holes could be really small diameter ones. They just need to be opened after each coating of separator with a fine wire. I'd be sending the ones without vents back to the manufacturer as not fit for purpose. If these moulds had vent holes, they would be a lot less bubble prone. 

Master moulds
If the mould continues to give trouble with bubbles, it might be best to take a negative of the mould that you can keep as a master.  Then make one-use investment moulds from this master positive as you need. Investment moulds usually allow air to move through the material pretty well, but you can add sprues if you want.

A further possibility is to drip the glass into the mould.  To do this you need to place a ceramic pot, supported by kiln furniture, above the mould with the glass for the casting in it.  Take to a temperature between 850°C and 900°C, depending on how long you wish to wait for the glass to flow out of the pot and into the casting mould.  The action of the glass forming in the pot eliminates many of the bubbles caused by frits and powders.  A further advantage is that this forming in the pot eliminates the possibility of the edges of the original glass pieces being seen. It would also allow you to add a different colour causing swirls or wisps of colour to move through the main colour.

The main effort is to eliminate the bubble formation.  This can be done with vents, adjusting the schedule, modifying the method by melting the glass into the mould, or making a master and individual investment moulds.  You can also combine several of these methods in one firing if you wish.

Wednesday, 21 September 2016

Firing Rates

Top temperature is, to a small extent, variable between kilns, even from the same manufacturer.  But it is a small part of variations in top temperature required to get the same results in differing kilns.

An example of a firing schedule

It is, more importantly, a function of how the heat is put into the glass. Firing as fast as possible to the top temperature does not allow all the glass to be at the same temperature. This is because glass is a good insulator and the transfer of heat from the top or the sides is relatively slow.  For small things, you can fire very fast, as there is a small mass of glass to absorb the heat.  But a speed of 250°C is fast enough for anything more than 100mm square and at least two 3mm layers thick.  (Thicker glass requires slower rates of advance as surprisingly do single layer projects).  The slower rate of advance allows the glass to be all of a similar temperature from top to bottom, allowing the desired effect to be achieved at lower temperatures or shorter soak times. 

For example, a slower rate of advance will give rounded edges at shorter soak times than a rapid rate of advance will require.  Alternatively, it might require a lower temperature with the same soak time.  Keep in mind that, in general, lower temperatures with slower rates of advance, give better results.

The faster your rate of advance, the more the glass lags behind the air temperature (which is what pyrometers are measuring). Therefore, a reasonable pace will give better results than the as fast as possible rate of advance. 

In short, the variations in top temperature required and length of soak is not about the kiln firing cooler or hotter as much as it is about the firing rate.

Wednesday, 14 September 2016

Adjusting Cut Runners

There are a number of types of cut running pliers.  These photos show some of them. 

The apparently most popular is this:


It is frequently difficult to find replacements for the plastic cushions that come with a new pair of cut runners.  People resort to a number of means to provide a substitute.  Some wrap electrical tape around the jaws, others use fabric bandages (Elastoplast/band aids).  I have even used the liquid plastic that is designed for coating tool handles.

However, if you adjust the cut runners appropriately, you can use them to run your scores even without cushions.  The purpose of these cushions is only to compensate for too much pressure in running the score.

Use without covers

You can run the score without cushions by using the adjustment screw on the top jaw of the tool. Yes, it does tell you which is the top jaw without having to check the end of the runners, but it has a more important use.  It is not just a pretty cool way to tell which is up. 

Its purpose is to adjust the width of the opening so that it provides the appropriate amount of bending force no matter how much pressure you exert at the handles.  If you are running scores in three-millimetre glass, set the jaws to that width by turning the screw until the jaws are that width apart.

Place the jaws at right angles to the score, aligning the mark on the top jaw with the score line and squeeze the pliers.  As you squeeze, the curved jaws provide enough bending force to run the score without over stressing the glass.  It is the adjustment screw that limits the over-stressing of the glass during the running of the score. Yes, you may not be able to run the whole length of the score this way, but you can repeat from the other end and that is usually enough to complete the running of the score.

You can continue to use cushions of various sorts with this adjustment for thickness, but I found that these were not necessary when the runners were properly adjusted.  In fact, I found that soft cushioning made more difficulties than using them with the bare metal.  I discovered this during the period of using the liquid plastic coating as used for tool handles.  I dipped the jaws multiple times to give a cushioning effect and it worked fine.  The cut runners continued to work even after the tool handle coating had worn off.  It was then that I realised I could control the running pressure more directly than by having a cushion between the glass and the jaws.

Setting the spacing

An easy way to set the correct opening of the jaws is to test against the glass you are about to score and break.  Place one side of the jaws against the edge of the glass. Slide that corner just a few millimetres over the glass.  Turn the set screw on the top of the jaws anticlockwise until they are fitting the glass snugly.  Back off a half turn (clockwise) so the jaws move easily along the edge.  This is now set to run the score on this glass. 

Open the jaws and place the centre mark in line with the score.  Close them gently and you can observe the arc of the jaws above the score line. Squeeze the handles and the score will run along the line away from the cut runners.  As you have adjusted the opening, no matter how hard you squeeze the cut runners, you cannot add more pressure.  This means you avoid crushing the glass.

The principles

The curve of the jaws is designed to provide the bending force required to run the score.  The radius of the curve has been designed to provide the correct bending pressure for differing sizes of glass.  The most common ones are useful for glass up to, but not including, 6mm glass.  The screw adjustment provides compensation for differing thicknesses of glass.  Setting the width of the gap to match the thickness of the glass prevents the application of too much pressure.

Thicker glass

For thicker glass you need cut runners with wider jaws.  These usually are fitted with three points to apply the breaking pressure - one under the score and one each side of the score on the top.  Again, these are adjusted to be just less than snug to the glass before applying the pressure.

One example of  cut runners for thick glass.  There are a variety of others.

Line Widths for Cartoons

The lines for copper foil and lead cartoons need to be of different sizes.  Only a small width is required between glass pieces in copper foil.  This allowance is for the two thicknesses of foil and a space for the solder to run through from front side to the back side.  In leaded glass a wider line is needed to allow for the width of the heart of the came.

On cartoons for different methods, draw the lines in the appropriate width.  For copper foil this width is ca. 0.8mm.  This can be accomplished with a ball point pen or fine felt tip. 

For leaded glass panels, a thicker line of ca. 1.6mm is required. A bullet tipped felt pen is usually appropriate, if it is not worn down at all.

A cartoon for fused glass should use the finest line possible, as the glass pieces will be in direct contact (ideally) with each other.  As in copper foil, a ball point or fine felt tipped pen will be appropriate.

Wednesday, 7 September 2016

Marker Residue on Glass

Often it is essential to make marks on the glass in preparing it for the kiln. However, sometimes these marks are visible in the final product. When making marks on glass in preparation for cutting or assembly in a fused piece, a balance needs to be struck between ease of cleaning and the retention of the marks as long as necessary. Often, when the marks are in spirit based markers, the temptation is to hope the marks will fire out without any further work. This is not a sound practice.

For the most temporary of marks use erasable markers, like white board markers. These will wipe away with a paper towel, leaving no marks after firing. These may not last long enough for your purposes though.

The next set of temporary markers are the permanent markers. These are more durable and resistant to being smudged off the glass. Most often they will fire cleanly away in the firing. But there are occasions when they don't. So it is best always to remove the marks before assembly. Usually water will remove the marks with a little rubbing. If not, then a spirit based agent will be needed. Of course then you need to remove the mineral spirit residues. I normally do this with window cleaner as used by glaziers, with no additives.

The most permanent marks are done by the paint markers. These do need spirits to remove them, or they will get fired into the glass. The removal of the mineral spirits is as for the permanent markers.

This example gives a range of colours.  It is best to use contrasting colours and I use black and white almost exclusively 

Of course, the best method of keeping marks off the glass is prevention.
In so far as possible:
  • Don't use permanent markers
  • Don't use oil in your cutter.

Temporary markers are usually all that is necessary.
Oil is definitely not necessary, merely a convenience, in your cutter.

[revised 07/09/2016]

Cutting from Cartoon

The cutting of glass directly over the cartoon without patterns is variously called trace cutting, English or European method. The advantages include a more direct process with fewer operations, making for less chance of inaccuracies.  These make this method quicker than using patterns or templates to draw or cut around. The disadvantages are that you still need to make patterns for opalescent or very dark glass, and there is no pattern to guide any grinding required.

To employ the method, use a strong contrast ink for drawing the lines of the cartoon. Draw these in the appropriate width - for copper foil (ca. 0.8mm).  This can be accomplished with a ball point pen or fine felt tip.  For lead, a thicker line (ca. 1.6mm) is required - a bullet tipped felt pen is usually appropriate, if it is not worn.

To cut, place the glass over the appropriate part of the cartoon and cut at the inside edge of the line. It is best to cut and break one line at a time.  Re-set the newly cut edge along the cutline and score the next line.  Break it and repeat the number of times required to cut out the whole shape.  Which line should be the first to be scored and broken is described here

Although the glass is normally only three millimetres thick, there is some possibility of a refraction of the light if you look from the side of the cutter.  As described elsewhere, you should be holding your cutter upright in the left to right plane and angled slightly back toward you so that you can look with your dominant eye along the barrel of the cutter and head.  This ensures you are directly above the cut line and do not have any refraction caused by the glass. 

Translucent glass may, and opalescent glass will, need a light box to assist in the scoring.  This increases the light coming through the cartoon and glass to enable a sharp image of the lines to be seen.  This means that if you intend using opalescent glass, your cartoon should be done on translucent paper to allow the maximum light through.

For black, dense and strongly opalescent glass, pattern pieces will need to be cut, as the insufficient light will pass through the glass to be able to see the cartoon accurately.

The reduction in the number of operations to guide the cutting of the glass also reduces the occasions for small errors to creep in.  It does increase the accuracy of cutting and speed of building a panel.

Wednesday, 31 August 2016

Scheduling for a New Slump Mould

Often you will see statements that imply a single temperature and time is suitable for all slumping or draping.  This is not so.  In fact, slumping is a delicate balance of layup, time, gravity, shape and temperature. This applies to draping operations too.

Factors in glass forming

The balance of colour arrangement has an effect on how the glass forms.  In an extreme example of white on one side and black on the other, the forming will begin on the black side first, leading to an uneven slump. Read on - there are ways to make this effect less severe.

The length of time you are willing to wait for the piece to slump is a factor in the temperature required.  Patience is rewarded.  Longer soaks mean that lower temperatures can be used. Lower temperatures lead to less marking on the back.

The mass (often thought of as the thickness) of the glass affects how quickly the glass will form. The greater the mass, the sooner the glass begins to form. This means with heavy glass lower forming temperatures can be used, because of gravity effects.

To get glass conform to a mould with complicated shapes takes longer soaks or higher temperatures than simple shapes.  This is because the glass requires to be more plastic to get into multiple shapes, small details or sharp angles.

General Principles

Since all these factors interact, any one schedule will not do for all occasions.  The general principles for a good slump are:
  • Use a steady rate of temperature increase (rate of advance).
  • Use the lowest practical temperature to get the forming done.

The reasons for using a single steady rate of advance in kiln forming are:
  • It is much simpler to program a single rate of advance all the way to slump temperature.  
  • Glass reacts best to steady inputs of heat, allowing the whole substance to be at the same temperature as it heats up.
  • It helps avoid uneven slumps.  Glass that is the same temperature across and throughout itself is more likely to begin to form all at the same time.
  • It helps ensure that the whole of the thickness of the glass is at the same temperature, thus avoiding splits on the bottom.
  • The slow steady input of heat means the glass can be formed at a lower temperature because of the heat work put into the glass on the rise in temperature.

The reasons for using the lowest practical temperature to slump and drape are:
  • It allows the glass to begin moving before it gets sticky, and so dragging on the mould producing stretch marks and sometimes needles.
  • A low temperature slump reduces the risk of uneven slumps.  At low temperatures the glass is less likely to react to colour variations that absorb heat more quickly than others.  Where there is uneven weight, the forming is more likely to be even as it cannot react so quickly to the differences in weight.
  • The glass will be less marked on the mould side at lower than at higher temperatures.  The glass, being less plastic, will take up less of the mould texture.

Calibration of Schedules

As each mould is different, there are as many schedules applicable as there are moulds.  Bullseye has recognised this by publishing suggested schedules for their moulds.  But there are lots more moulds than the Bullseye ones.  And even for the Bullseye moulds there are a variety of variables in the glass put on top.

The point is to find a way to determine the appropriate schedule for the mould and the glass it supports.  This involves the main variables - rate of advance, top temperature and soak time - although there are others such as lay up, degree of fusing, weight and its distribution, colour variation, etc.

The rate of advance will depend on:
  • The thickness of the piece.  Thicker glass needs slower rates of advance.
  • The degree of fuse.  A tack fused piece will require a slower rate than a full fused piece.

The top temperature depends largely on the complexity of the mould shapes, although it is very closely related to the soak time.  One of the principles of slumping given above is to use the lowest practical temperature. The reason for this is to get a good result with the minimum of mould marks.

The main means of determining forming temperature and time is observation. I determine my slump temperature (normally) by what temperature I have to use for the particular mould to get the glass fully slumped in half an hour.  For more complicated moulds such as a candle bridge I would use 1.5 hours as the soak time.

There are two main methods of doing this observation.  One is to set the “one size fits all” schedule and modify it. The other is to create a new schedule by working up from the lowest temperature to the practical temperature.

Modification of existing schedule

To prepare for the modified schedule, you need to do several things.  

Get your kiln log out ready to record the information about the firing.  Record the mould shape and separator (and add a picture of the set up if you can) and include the lay up of the blank to be formed.  Also record anything you think may be relevant to the forming process for this firing.

Set your single rate of advance all the way to the top (forming) temperature and record it in the kiln log.  Begin observing the progress of the slump from 60°C below the top temperature you have set.  This involves quick peeks at approximately five to ten minute intervals.  You may not see much movement at first, but at later peeks you will see the glass progressively forming.  When the glass appears to have just touched down at the bottom, you can use that as the top temperature.  Advance the schedule to the soak portion (read the controller manual if you do not already know how to do that).  Note the temperature and time in your log book when you do this.

Continue to observe the progress of the slump but at about ten minute intervals to check on the progress of the slump.  When the slump appears complete, advance to the next segment of the schedule and note the time.  Subtract the start of the slump soak from the present time and you know how long the soak needs to be for that layup in the mould. Record that in your log book. 

When cool, inspect the slumped piece to determine if it is fully formed. Record the results in your kiln log.  If it is not fully formed, you can decide if it is practical to add additional soak time or if you need to increase the top temperature.  Only you can determine what is a practical soak time.  If you are soaking while you are away or asleep, it does not really matter how long a soak you need at the chosen temperature.  However, there are times when you need to have a piece out of the kiln to be able to put in the next.  Somewhere between these two is the practical soak time.

You may find that the glass does not need as much time as you gave it.  Record this result too.  In this case, you can reduce the top temperature in future firings until you find the best combination of temperature and time. You will have experience from watching the forming (whether slump or drape) to give an indication of the lower temperature to choose.  A general guide would be to reduce the temperature by 10C, and extend the time by at least 50% more than  what you used in the higher temperature firing.  

Record each firing with the lay up, rates, temperatures and soak times, plus the results.  When you have determined the ideal combination of factors, record the determined temperature and soak time together with the layup in your log book and on something in your mould box.  I have also used vitreous paint on the underside of the mould to indicate my standard temperature and soak time so that I don't loose the information.

Development of a new schedule

This is not as difficult as might be imagined.  It does involve a lot of peeking into the kiln, though. You start with an appropriate rate of advance for the thickness and style of fusing.  Remember that thicker glass and tack fused glass require slower advances than thinner and flat fused glass.  Set this rate all the way to your predicted top temperature.  No rapid rises with short soaks are required or desirable. Set a predicted soak time. If you are not certain, use 30 minutes as a general average. Then set the anneal soak and cool rates.

As you observe, you will see when the glass on the mould begins to form. It will generally start at about 600°C.  Peek at about 10 minute intervals from that temperature onwards toward the target.  When you see the glass begining to change shape, Change the top temperature to be about 20C higher than the initial forming temperature, and then observe after 15 mins at the new temperature. If it hasn't moved much, add 5°C more to the temperature and observe. Repeat as necessary. When the glass has a significant curve, stop the rise and soak at that temperature with the 30 minute soak.  Continue to observe at 10 – 15 minute intervals to determine when the slump is complete.  Then proceed to the anneal cool. Record rates, temperatures and times in your log book.

When you remove the piece from the kiln, check it over.  If it is not fully slumped, you can add time or temperature.  Adding time is likely to give a better surface to the glass on the mould side.  Sometimes, but not often, adding temperature will be the choice. 

It is possible that the piece will show evidence of too high a temperature or too long in the mould.  This will be clear from extensive mould marking, sometimes needles at the edges, stretch marks, or uprisings at or near the bottom of the mould.  In these cases, the temperature needs to be reduced.  Reducing the time is not advisable, as quick slumps can often be distorted or unbalanced.

Glass Types

Remember that these tests for the best forming schedule for you and your mould are only relevant to the kind of glass you are using at that moment. There will be only minor variations between Bullseye, Uroboros and Wissmach. There will be major variations between these and float glass. Separate schedules will need to be worked out for it, remembering that there are a variety of manufactures of float and they do not all behave the same as each other.  Float and other glass that is not formulated for fusing will not provide such consistent results as fusing glass, but successful schedules can be determined in the same way as for the fusing compatible glasses.


Once you have calibrated the temperature and time for the mould and the layup, you will know how to schedule for that mould. Record it in your log book and also along with your mould, either in the box or on the mould.

It will be for you to decide whether you use longer times and therefore lower temperatures.  When making the decision remember the principles of slumping – steady rate of increase to the working temperature, and use of the lowest practical temperature.

These actions will give you the standard forming temperature for the mould.  It is a base from which to make variations when you use a different thickness, lay up, or degree of fusing.  

You should continue to record each of your firings with full details, because sometimes things change. This will give you a basis to diagnose what has become different. It will help avoid the cry of "this has always worked for me before."  It means you have the possibility of working back to see what, if anything has changed. If nothing has changed in your level of fusing, thickness, lay up, schedule and all those other things you record, then you can begin looking at your kiln to see what might be different.

Wednesday, 24 August 2016

Kiln Forming Myths 30 – Slump Depth

You can slump only 5cm or so per firing. If so, why don't drop rings fail?

This is really about a comparison of deep slumps and free drops.

Multi-stage Slump
·         The stages of a deep slump give more control of the design
·         The shape of the result is determined by the shape of the mould
·         The wall thickness will be much more consistent
·         For its size the deep slump is lighter than the drop. 
·         The multiple stage deep slump is designed to have the minimum of cold work.

Single Stage Slump
·         If you attempt to do a deep slump all at once you run the risk of ruffle around the upper edge. 
·         There are likely to be a large number of stretch marks on the outside.
·         The design will be distorted to varying degrees along the surface.
·         The wall thickness will vary greatly.
·         You lose the advantages of the multi-stage slump without gaining the advantages of a drop vessel.

Drop Ring Vessels
·         Drop rings require higher temperatures. 
·         The glass thins as it stretches through the ring, so you need to start with a much thicker blank than slumping. 
·         There will be a thick base in relation to the sides.
·         The design will stretch, and if properly designed will be very pleasing.
·         The walls of a drop vessel will vary from thick at the bottom to thin at the top.
·         The collar needs to be cut off the vessel and cold worked to smooth.
·         The process of falling through the ring needs to be monitored to avoid an excessive drop causing distortion or an insufficient drop causing the need for grinding a flat base for the vessel.

Wednesday, 17 August 2016

Reversibility of Boron Nitride

After using Zyp/MR97, can I sand it off and use kiln wash?

Some people are applying boron nitride to ceramic moulds for the "smoother" surface.  Boron nitride is an excellent separator for metal moulds and casting moulds whether metal or ceramic. But it has limitations, including the price and requirement for a repeated application at each firing.  Some are beginning to wonder if they can go back to kiln wash after having used the boron nitride. Some say you cannot unless you sand off the separator.

The general experience has been that you can't apply kiln wash on top of the boron nitride. It just beads up and flows off, because the boron nitride creates a non-wetting surface that survives relatively high temperatures.  The water in the kiln wash mixture merely beads up or washes away. This means the kiln wash in suspension has no opportunity to adhere to the mould.

The most accepted way to get rid of the boron nitride is by sandblasting. Then apply kiln wash as normal. The sandblasted ceramic mould previously coated accepts kiln wash with no difficulty. In the absence of a sandblaster, you can use a sanding pad. You do need to be cautious about taking the surface of the mould when using abrasive removal methods, as the ceramic is relatively soft in relation to the abrasive materials.

The difficulty of removal of the boron nitride means that you have to think carefully about which moulds you put it onto.  If the mould has delicate or fine detail, removing the boron nitride risks the removal of some of the detail.  This indicates that this kind of mould, once coated, should not be taken back to the bare mould to change the kind of separator.

Another use of boron nitride is to spray a very small amount on a fiber strip to be used for damming. This will give you fewer needles as it provides a non-wetting surface at relatively high temperatures. This allows the glass to slide down the fibre paper without hanging up and creating the needles.

One advantage of kiln wash over boron nitride is that you do not have to reapply every firing as with boron nitride. With the boron nitride you need to apply before every firing.  It is best to use a paint brush to dispose of any lose material before giving a light re-coating. Not a whole lot is required on subsequent coatings.

If you are using boron nitride to get a smoother surface to the object, consider using a lower slumping or draping temperature, as this will also minimise mould marks.  

Wednesday, 10 August 2016

Kiln Forming Myths 29 - Super Glue

The use of super glue in the kiln causes cyanide gas
This is not true.  But because it is such a persistent belief, a lot of detail is given below.  In short the precautions are: 

  • use the minimum amount, 
  • use an organic gas face mask, 
  • do not wear natural fibres or gloves, 
  • let the glue cure before placing it in the kiln, 
  • have the solvents at hand while using the glue.

Super glue is frequently used as a temporary fixative in assembly of kiln forming projects. There is some concern about safety, as it is known that super glue is made from cyanoacrylate, which it is feared will break down in the kiln into cyanide gas.

Greg Rawls, a certified industrial hygienist says

"I looked at the MSDSs for several forms of super glue. The main component is Ethyl 2-cyanoacrylate, which has a TLV of 0.2 ppm which is relatively toxic. [However,] the thermal decomposition products are carbon monoxide and carbon dioxide. I did not see a reference to cyanide gas. However, as I recall cyanide gas dissociates into elemental carbon and nitrogen at about 800 F. Since you use it in such small quantities, I would not worry about it. In my opinion the worst thing that could happen is you glue your fingers to the glass."

Safety issues

To treat the safety issues seriously and determine if you feel Greg Rawls' view is justified, you need to look at the issues of toxicity, reactions, adhesion of tissue, ventilation, first aid and decomposition products in the whole context.

The fumes from cyanoacrylate are a vaporized form of the cyanoacrylate monomer that irritate sensitive membranes in the eyes, nose, and throat. They are immediately polymerized by the moisture in the membranes and become inert. These risks can be minimized by using cyanoacrylate in well ventilated areas. About 5% of the population can become sensitized to cyanoacrylate fumes after repeated exposure, resulting in flu-like symptoms. It may also act as a skin irritant and may cause an allergic skin reaction. On rare occasions, inhalation may trigger asthma. There is no single measurement of toxicity for all cyanoacrylate adhesives as there is a wide variety of adhesives that contain various cyanoacrylate formulations.

The United States National Toxicology Program and the United Kingdom Health and Safety Executive have concluded that the use of ethyl cyanoacrylate is safe and that additional study is unnecessary. 2-octyl cyanoacrylate degrades much more slowly due to its longer organic backbone that slows the degradation of the adhesive enough to remain below the threshold of tissue toxicity, so the use of 2-octyl cyanoacrylate for sutures is preferred.

Reaction with cotton

Applying cyanoacrylate to some materials made of cotton or wool results in a powerful, rapid exothermic reaction. The heat released may cause serious burns, ignite the cotton product, or release irritating white smoke. Users should not to wear cotton or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.

Adhesion of the Skin

Various solvents and de-bonders can be used. These include:
·         Acetone, commonly found in nail polish remover, is a widely available solvent capable of softening cured cyanoacrylate
·         Nitromethane
·         Dimethyl sulfoxide
·         Methylene chloride
Commercial de-bonders are also available.

Warnings include:
·         It is a mild irritant to the skin.
·         It is an eye irritant.
·         It bonds skin in seconds.
·         Any skin or eye contact should be copiously flushed with water and medical attention be sought immediately.
·         Do not attempt to separate eye tissues – the bond will separate naturally within a few days.

·         Use goggles.
·         Do not wear cotton or wool clothing while using super glue
·         Ventilate the area well. Since cyanoacrylate vapours are heavier than air, place exhaust intake below work area. Activated charcoal filters using an acidic charcoal have been found effective in removing vapours from effluent air so the bench top air filters are suitable for use while using super glue.
·         Avoid use of excess adhesive. Excess adhesive outside of bond area will increase level of vapours.
·         Assemble parts as quickly as possible. Long open times will increase level of vapours.

Evaporation Effects
·         The effects of heating cyanoacrylate are not completely known. The flash point is known to be greater than 85ÂșC. As a precaution do not remain in the area of the kiln after that temperature has been reached.
·         The decomposition products are carbon monoxide and carbon dioxide. There is no reference in the literature to cyanide gas. It is highly unlikely that heat will cause the release of cyanide gas at any time during the heating. To be certain, you should make sure the evaporation of the glue is complete before firing the kiln.

See this tip for the use of super glue in kiln forming.