Wednesday, 23 September 2020

Making Thin Sheets

The question of how to make thin sheets arises from time to time.  Unless you are a glass manufacturer, it is unlikely you can make large, thin glass sheets.  But you can approximate making thin sheets by two methods that I know.


One of these is sintering.  This is firing the glass to a low temperature and soaking for a long time.  The common form of this is powder wafers. 

By using a screen to deposit an even layer of glass powder you can make very thin, but delicate sheets of glass.  The procedure I would use is a screen of about 45 – 60 threads per inch.  This is coarse enough to allow the powder through, but not so fine as to “reject” large amounts of the coarser particles. 

You can screen the powder directly onto a kiln washed shelf, or onto Thinfire or Papyros.  You will not be able to move the unfired powder on a sheet of paper or fibre paper without changing the thickness and shape of the screened powder.  It must be laid down onto the separator directly on the shelf.  You can of course, move the shelf to the kiln if you can get in without tipping it.


Support the screen about 3mm above the surface to allow the powder to fall through.

Make a ridge of powder at one end of the screen.  Using a smooth straight edge wide enough to cover the whole of the screen, lightly spread the powder from the starting end to the other. Then repeat drawing the powder to the starting end.  Make about five repeats of this – that is 10 passes, to get enough powder laid down to form about 0.5 to 1mm sheet.  You will need to experiment with the number of passes to get what you want.

Do not try to press the powder through the screen.  That will only wear the screen out quickly and may tear it.  Each pass should be a light spreading of the powder.  It is heavy enough to fall through the screen without additional force.

You could, of course, just sift the powder over the area you want to cover and judge by eye how even the layer is.  It is possible that your observation is good enough, but it is more likely that you will have thick and thin areas.  Often even at sintering temperatures, the thin is pulled toward the thicker, leaving small or large holes.   By screening the powder, you know you will have an even layer


The kind of schedule to use to sinter the glass particles together without changing their structure is the following:
220°C to 482°C , soak for 60 mins
55°C to 593°C, 10 minutes
28°C to 665°C for 5 mins
as fast as possible to 482°C for 30 mins
28°C to 427°C, no soak
55°C to 370°C, no soak
110°C to 50°C, no soak
This will work for most fusing glasses.

This slow firing allows enough heat to penetrate the glass grains that they will stick together without changing shape or developing holes.  I admit the anneal cool is very cautious.  You can experiment with quicker cools if you want to speed the process.


This is a technique of thinning already existing sheets of glass.  Typically, you will have a 6mm or thicker piece of glass that you want to be 3mm or less.  Paul Tarlow has described this kiln pressed glass very well in his books and on the site.

In essence, you use a pair of kiln shelves.  Kiln wash both shelves.  Place the glass to be thinned on one shelf.  At the outer edges of the shelf put down spacers of the thickness you want the glass to be after pressing.  This will keep the upper shelf from settling down too much and more importantly unevenly.  Place the other shelf, kiln washed side down, on top of the glass.  Be sure the spacers are in places where they can support the upper shelf.

If you are thinning from 6mm to 3mm, normally you do not need any additional weight on top of the upper shelf.  But the thinner you want the glass to be, the greater the weight needs to be.  It could be another shelf, fire bricks or steel weights.

When scheduling the annealing remember you must take account of the mass of the weight on top of the glass.  You will need a much longer temperature equalisation soak and a much slower annealing cool.  Details for this whole procedure are in Paul Tarlow’s tutorial referenced above.

Wednesday, 16 September 2020

Keeping Bottles from Rolling

A common problem in firing bottles is that they may roll into one another and stick, making both bottles useless.

One way to overcome this is to let the bottle find its heavy point by setting on smooth and level surface. It will gently roll to one direction before slowly coming back in the other. When it stops this oscillation, the heaviest part of the bottle will be on the bottom.  Mark the bottle in some way so you can move to the kiln in that position. If after this, it rolls in the kiln, then your shelf is not level. 

Additional assurance against rolling is putting a small piece of thin fibre paper (1 or 2 mm)at each side of the point the bottle touches the shelf.  Thinfire and Papyros are not enough to ensure there will be no movement. But the small bumps of fibre paper are enough to stop the bottle from rolling.

Sometimes you want a particular part of the bottle up or down, but it won’t stay in place.  Then you need to put a slightly thicker piece of fiber paper against the bottle on each side.  It is better if it is not Thinfire or Papyros as they tend to disintegrate above 400C, long before the bottle begins to distort enough to keep it in place.

Other materials you can use to prevent the bottle from rolling are crumbled chalk, whiting, kiln wash, or even a few grains of sand.

Preventing bottles from rolling in the kiln is about finding the natural heavy spot, or propping the bottle in place with a variety of heat resistant materials.

Wednesday, 9 September 2020

Long Annealing Soaks

You Can’t Anneal Too Long.

Can you anneal too long?

Yes, you can.

It’s not just the possible temperature differences in the kiln.  If you have temperature differentials across your kiln, any piece that crosses those boundaries will have temperature differences locked into the glass.  If you know you have temperature differentials and your glass by circumstance must be in both the cooler and the hotter regions, you need to do a standard length of soak only.  Then reduce the rate of cooling a little more than normal, so that a slower cool occurs.  This should avoid most of the stress that can be induced by very long soaks in a kiln with hot and cool spots.

The other factor against annealing too long has been revealed by Bullseye research on annealing.  This video at about 13:00 minutes into the film explains.  This complicating factor in annealing is about the difference in temperatures of the surfaces of the glass.  The research shows that the longer you anneal the greater the differential in temperature becomes between the upper and lower surfaces of the glass.  This means that you can introduce stress across the whole piece, rather than just a section as in an unevenly heated kiln.

What is more, the longer you soak, the cooler the bottom becomes in relation to the top.  The reported research does not state the reasons for this.  It just commented this as an observational fact.  It can be assumed that the air temperature differences are the cause.  Even during cooling the air is hotter on top of the shelf than under.  This would allow the bottom surface to cool more than the top. This assumption is borne out by the fact that the effect is reduced or eliminated by having elements under the shelf.

There are two reasons to avoid long soaks. Uneven temperatures across the surface are locked into the glass.  And long soaks at annealing induce an unwanted temperature differential between the top and the bottom of the piece.

Wednesday, 2 September 2020

Altering Annealing Temperatures

Sometimes  it is discovered that a kiln is firing hotter than other kilns, and you need to alter your process temperatures from the generally presented ones.  That your kiln is firing hotter than others is when you recognise the tack fusing profile of your tack fused piece is rounder than expected. 

Altering process temperature and soak times

There are two things you can do.

1)  Reduce the time at the temperature.  If the recommended schedule has the process work being done at 780°C for 15 minutes and the glass is too rounded or more like a contour fuse, you can reduce the soak time to 5 minutes, depending on how over-done the pieces are. 

2)  If the reduction in soak at process temperature does not work, then you can begin to reduce the process temperature.  Often only 5°C with a 10-minute soak is enough.  For some kilns it may be as much as 20°C again with a 10-minute soak.

Remember that the speed at which you advance to the process temperature will have an effect.  The slower you go the lower the temperature can be.  The faster you go, generally the higher the temperature needs to be.  There several factors combining to determine which is the right process temperature and soak.  Experimentation and record keeping are required to find just the right combination.

Annealing temperatures in a “hot” kiln

If your kiln fires hot, you do not need to alter the annealing soak temperature.  I have seen the recommendation that when you need to reduce the process temperature you also need to reduce the annealing temperature by the same amount.  This is not so for several reasons.

The first is that reducing the temperature of the annealing soak runs the risk of trying to anneal below the acceptable range.  These are a few paragraphs to explain.

Annealing occurs over a range.  The annealing point is the temperature at which annealing can most quickly occur.  But there is a range during which annealing can occur.  It is generally around 43°C either side of the annealing point.

If you follow the recommendations to anneal in the lower end of the annealing range, it is possible that you will start the annealing soak at too low a temperature by reducing the annealing soak temperature in line with the reduction of the top temperature.

The second is that the temperature measurement is of the air, not the glass.  On cooling, the glass is hotter than the air temperature in the kiln.  The recommendations for the annealing temperature take that into account.  So, reducing the temperature risks straying outside the annealing range.

Example of the annealing of a tack fused piece comparing temperatures of the air to the under tack stack and exposed base during the anneal soak and first cool

You should note that if you are using the Bullseye recommendations to do the anneal soak at 482°C, you already are in the lower end of the annealing range.  The average annealing point of Bullseye remains at 516°C. This new recommendation for the annealing soak is 34°C below the annealing point and any reduction of more than 9°C will put your anneal soak outside the annealing range, meaning that your anneal will be inadequate, no matter how long you soak there.

The third element relates to the annealing range.  The anneal soak can occur anywhere within that range. But the practical measure is to soak at, or below, the annealing point.  If your kiln fires hot, you do not need to alter the annealing soak temperature.  It will not matter if the glass is in fact hotter at the annealing soak than in some other kilns. 

It does not matter, because the soak at the annealing point, or lower in the range, is to equalise the temperature throughout the glass piece. The annealing point is not some magic number or temperature that sees to producing a sound piece of glass.  The soak at annealing point is to equalise the temperature to + or - 5°C within the glass.  This is referred to by the technically minded as Delta T = 5°C, or in symbols as Δ T = 5°C.  Bullseye has published a table that gives practical information on the length of soak required for this temperature equalisation for different thicknesses.

Once the temperature is equalised within these limits, you can begin the anneal cool.  This is an essential part of annealing and is designed to maintain the equality of temperature differentials during the cooling.  The rate of cooling is directly related to the length of the temperature equalisation soak required for the piece which in turn is related to the thickness of the piece.  This forms the fourth reason that starting the anneal soak slightly higher than recommendations, will not affect the annealing process adversely. The first slow cool is essential to achieving a sound piece as it maintains this small differential in temperature during the early part of the cooling into the brittle phase of the glass.

Annealing Temperatures in a Cool Kiln

Exactly the same reasoning process is applied to both hot and cool firing kilns.  You do not need to alter the anneal soak, even though it means you will start the temperature equalisation at a slightly lower temperature than the published schedules.  This is because you have to increase the top temperature to get the effect you want and so would also be annealing in a cooler kiln.  Since you are measuring the air temperature, the glass temperature will be above the air temperature and will still be in the safe annealing range.


The reasons annealing temperatures do not need to be altered if you kiln fires hot or cool are related to:
·        annealing range
·        air temperature measurements
·        rate of the anneal cool

Wednesday, 26 August 2020

Uneven Slumps

A common problem in kilnforming is that the glass slumps into the mould unevenly. Several of reasons are given in this post about high temperature or fast slumps for uneven results.

There are two other things that can be done to alleviate uneven slumps.

Place the mould in the centre of the kiln to reduce any uneven heating of the glass.  Uneven heating is a common cause of off-centre slumps.  Where you have persistent uneven slumping with a mould it may be better to fire it on its own so the conditions can be best for it.  Sometimes it is more economical to fire a single item rather than a crowded kiln shelf where the firing conditions must be for an average rather than the optimal firing schedule and conditions for one mould.  Less of the resulting slumped glass is disappointing.

There is an alternative. Cut the glass so the fused piece will be slightly smaller than the mould top. This will allow the glass to sit inside the mould rather than on top. Frequently there is evidence of the glass hanging up on the side of a mould.  Sometimes there are spikes where the glass stuck and stretched. (Another reason for Low and Slow)

A third method has been suggested, but I have not tried it.  This is to lightly bevel the underside of the piece to be slumped.  The basis for this suggestion is that a bevelled edge will fit the mould better by having a slope rather than a relatively sharp edge resting on the mould surface.  I do know the other two suggestions work, but not this one, although it sounds logical.

Wednesday, 19 August 2020

Slumping sizes

A question about slumping into a bowl mould.

·        Will the end result always be that the glass drops down into the mould and so the bowl size is smaller than the mould?
·        Does the firing schedule affect the outcome?

You need to think about what is happening to the glass in the slumping process to understand what the outcome of a slump will be.
During slumping, the glass is being bent rather than stretched (very much) into the mould.  In other, higher temperature processes the glass is being both stretched and thinned.

The reason for using low temperatures is to reduce the thinning effect while obtaining the shape of the mould.  If you use higher slumping temperatures, the glass will slide down the mould more than at low temperatures, and will begin to thicken, or create an uprising, near the lower portion of the mould.

Take the simple case of a ball mould. One which has no flat bottom but a simple curve from edge to edge.

If the glass was 300mm diameter when flat, it will still be (approximately) 300mm from edge to edge on the bottom.    If you put a flexible tape measure around the bottom from one side to the other, it will be very near the 300mm diameter around the curve.

This means the new diameter of the rim will be smaller than the flat diameter.

Steep moulds have less change in diameter, but greater change in the height of the piece in relation to the mould.  Again, the glass measured from side to side on the bottom will be about the same as the flat piece.  To achieve this, it will slide down into the mould more than a shallow one.

Wednesday, 12 August 2020

Anneal soaks

An odd concept was presented recently.  This in summary was that if you have long soaks on the way up to top temperature you do not need to have such a long anneal soak as normal.

This is a fundamental misunderstanding of the physics of glass.  As the glass temperature rises above the upper strain point (about 55°C above the annealing point), the molecules become disordered.  No amount of soaking at any temperature on the way up to the top temperature will change that. 

The glass (and the molecules of it) will need to be cooled relatively quickly from the top temperature to avoid crystallisation of the glass.  This is the reason for the fast cool to the annealing soak.  It is also a reason to avoid a soak at approximately 50°C above the annealing point – there is a slight risk that crystallisation could form.  This would appear as scum marks on the surface, rather than in the interior.

Whatever soaks you have performed on the way to top temperature, you will need the full length of soak for the full or tack fuse.  And you will need it for the slump too.

No amount of soaking on the way up to top temperature in kilnforming will have any effect on the requirements for the annealing soak at the cooling part of the schedule.  The soaks in the early part of the schedule, no matter how many or how long, do not change the annealing requirements.

Wednesday, 5 August 2020

Convex Shapes for Wall Hangings

The most common shapes for wall hangings seem to be the “S” or wave form in various sizes, and flat pieces of what ever outline supported by stand-offs.

There is another possibility.  You can produce a shallow domed shape which can work well for either landscapes or abstract pieces.  They will be best if circular, although rectangular forms can be used.

The usual resistance to doing this is that the surface will be marked, or that the tack fused surface will be flattened.

There is a way to do this without either effect.  Place the work upside down on a mould of appropriate diameter or dimensions and fire the piece slowly to a low temperature. 

Raise the temperature more slowly than you usually would for a slump in the normal way – top side up.  This allows both surfaces of the glass to be at the same temperature at the same time.  This equalisation of heat throughout the piece will protect against any breaks or splits on the underside of the glass – which will become the top surface.

Set the temperature for about 620C, depending on the span of the piece.  This temperature will be suitable for pieces of 300mm to 400mm and 6mm to 9mm thick.  Pieces with a smaller span will require higher temperatures or longer soaks.  Larger pieces will need a lower temperature.

You should set the soak at about 45 minutes. You will need to observe at intervals until you have the amount of depression you wish.  You will also need to know how to advance to the next segment of the schedule when that point is reached, so that you do not over slump the piece.

Since the piece only touches the mould at the rim, and you are not allowing much movement in the glass, you will not mark the glass with the mould. 

This process of making a domed wall piece will be unusual, although it will not be appropriate in all circumstances.

Wednesday, 29 July 2020

Measuring for Circle Cutting

Often there is uncertainty about which way the cutting head should be placed on the bar of the circle cutter to get the right diameter.  And the distance markings on the arm often get worn away.

It is for these two reasons that I have given up trying to get the right diameter circle from the measurement markings on the arm of the circle cutter.  Instead I measure and mark out the centre point and the radius of the circle directly onto the glass.  Only a few tools and supplies are needed.

Glass, measuring stick, marker pen, oil and circle cutter are all that are needed to measure the circle

First you need to decide on the centre point, leaving at least 2cm at the edge of the piece the circle is being cut from to allow a clean circle to be broken out.

The four black does are for measuring from the edge to the axis

Once you have done that, mark an axis at right angles at the centre point.

This shows the axis established and the radius marked out on the left.

Measure the radius from centre line , mark that on the line. 

Place suction cup at the centre of the axis. In the case of the cutter I use there are four markings to assist in the centering of cutter.

Move the cutting head along the arm until the wheel sets right on the radius mark.  I find that getting low helps a great deal in seeing the placement of the wheel.

Tighten the locking nut.

I put a drop of cutting oil on the wheel, so that in a preliminary run, I can both see where the scoring line will be and be sure everything is far enough away that the arm does not hit something on the way around.

Score the circle, making sure your fingers are only on the knob. If your fingers slip down, they can loosen the locking nut.  Some people score in an anti-clockwise direction to ensure they do not loosen the locking nut.  An anti-clockwise motion means that if your fingers do touch the nut, it will be tightened rather than loosened.

This photo shows the circle scored and to show the spacing between the edge of the glass and the score line.
Once you have set the cutting head on the arm of the cutter, you can cut as many circles as you wish of the same size without needing to do further measurements.
Further information on breaking out the circle is given in this blog post and a more comprehensive guide to measuring and placing all sorts of sizes is given in  Drilling Glass, guide no. 7.

Wednesday, 22 July 2020


Crazing appears as the multiple cracks similar to what is seen on ceramic glazes.  These occur when there is a great deal of incompatibility between the glaze and the clay body.  This can also be seen in glass.

Crazing as seen on a ceramic object

I have see crazing of glass in two circumstances.  It happens with severe devitrification, to a maximum extent of crumbling under light pressure.  This usually happens with glass not formulated for fusing, and especially on opalescent glass.

The more common occurrence is where the glass has stuck to the supporting structure.  This is frequently the case where the separator has not been sufficient to keep the glass from sticking to the shelf.  This will happen on kiln washed shelves when the coating of the separator has not been even, leaving areas with bare or very thin areas.

The standard of mixing kiln wash in the ratio of 1:5 parts by volume of powder to water is important.  The application should be with a wide soft brush such as a hake brush.  The kiln wash should be painted on in four coats, one in each direction of up, down, and the two diagonals.  A well coated shelf should have an even appearance of the coating.  Only an even film of separator is required to keep the glass from sticking to the shelf, mould or other kiln furniture.

Wednesday, 15 July 2020

Adjusting Cut Running Pliers

Typical cut running pliers

Cut running pliers are very useful tools if used correctly.  The pliers must have the curve in a “frown” rather than a “smile” to operate properly.  The knurled screw at the top and the scored line on the top jaw help place the pliers the right way up. They must be placed directly in line with the score. They should be only a centimetre or so onto the glass.  Holding them at the end of the handles, apply gently increasing pressure until you hear a click or see the score running.  If it does not run completely, turn the glass around and apply the pliers to the other end of the score.

Use of the Adjustment Screw

It is important to make use of the adjustment screw to get the best from the pliers.  If this is not adjusted properly, it is possible to crush the glass, or at the other extreme, not run the score at all.

The jaws need to be adjusted for the thickness of the glass.  The method I use for this is to place the edge of the runners on a corner of the glass to be scored.  

Loosen the screw until the glass is gripped by the jaws.  
Gradually tighten the screw until it resists your gentle pressure on the handles.  This gives you the correct opening of the jaws for that piece of glass.

When the pliers are properly adjusted to the thickness of the glass, you will not crush the glass and it is easy to use the pliers without cushions.

Wednesday, 8 July 2020

Containing Stress

People frequently report success in combining incompatible glass pieces with a larger, different base.

Questions arise.

Have the resulting pieces been tested for evidence of stress with polarised light filters?

Other destructive methods such as hot water, or placing in the freezer are not adequate measures of the long-term effects of incompatibility stress.  When you are doing something outside the accepted norms, then you must test for stress to be certain what you are producing remains sound before announcing success.

Why does glass with incompatible pieces survive?

Incompatible glass will show some stress when viewed through polarised filters. You will need to decide when it is excessive.  When viewed between polarised light filters high stress will be shown by a rainbow effect in the halo of light.  Lesser stress will be shown by pale light. The degree of stress will be shown by the amount of light.


There are some circumstances where the glass can contain the stress, and others where it cannot.

Generally, large mass pieces can contain the stress from small incompatible pieces of glass. 

Spherical objects can contain a lot of stress over a long period, which is why glass blowers and lamp workers are generally less concerned about incompatibility than kilnformers are.

Flat glass pieces behave a little differently.

Circular forms can contain stress more easily than other shapes.  Rectangular  shapes generally show the most stress at the corners.  Narrow or wedge-shaped pieces have the most difficulty in containing stress.  The stress is concentrated at the points.

The placing of the incompatible glass is also important to the survivability of the glass.  The further from the edge of the piece, the less likely there will be breaks. 

The smaller the pieces of incompatible glass in relation to the whole, the less risk of breaking. 

The more spread apart the pieces are, the greater the chances of survival for a while or long term.

The most essential piece of equipment for people starting out and those who are investigating new setups or working at the edges of accepted norms is a pair of light polarising filters to test for stress.

When combining incompatible glasses the general case is that the greater the mass of the whole object in relation to the incompatible glass, the greater the chance of survival.