Wednesday, 20 May 2015

Firing Quickly

Firing quickly is often our desire, in spite of the mantra of the experienced – slow and low. How to do this safely – without fractures or bubbles – is the requirement.

Firing quickly on smaller things (say, up to 100 mm) is not normally a problem.  Difficulties can arise due to the kind of layup, but usually the mass is not great enough to be thermally shocked, nor the size enough to trap air that would cause bubbles.

Firing quickly for larger pieces is where difficulties arise. These relate to the initial rate of advance to the softening point, the bubble squeeze, advance to top temperature, and annealing.

Advance to softening point
The first place this occurs is to the upper strain or softening point.  This is the range where the glass is solid and does not transmit heat well, leading to the risk of thermal shock.  You need to find a rate of advance that is a little slower than that which would cause the glass to break.  My guidance is to use no more than three times the annealing rate for the glass of that thickness to reach the softening point.  This temperature is approximately 40C above the annealing point. The glass is certainly plastic above that temperature, so the rate of advance can be faster.

Bubble squeeze
Strategies from the softening point to bubble squeeze vary.  You can go quickly, say 1.5 times the previous rate of advance, to the bubble squeeze heat soak of around 30 minutes.  The other is to go quickly to 50C below that temperature and advance at 50C per hour to the bubble squeeze heat soak.  This is often used on more complex and thicker lay ups. There are numerous variations upon these two strategies depending on the circumstances.

Top temperature
Ways to get to top temperature from the bubble squeeze vary, but as fast as possible risks bubbles due to excessive softening the surface, over firing due to the controller not shutting off quickly enough, and a lack of control of the surface texture.  Twice the initial rate of advance is quick enough, but still allowing the controller to shut off when the top temperature is being reached.

The s
oak at top temperature does not need to be more than 10 minutes.  If you can achieve the desired results in less that time, you should consider reducing your top temperature.

Annealing cool
A s
oak at the bottom end of the annealing range will reduce the anneal cooling time.  The lower temperature of the annealing range is about 40C below the annealing point, so to be safe the annealing soak can be set to be 30C below the annealing temperature.  This reduces the range of temperature over which the slow anneal cool takes place. 

The initial anneal cool should be to 55C below the soak, the second stage of the anneal cool to 110C below the soak can be at twice the initial anneal cool rate.  The rate of cool can be increased to 370C, where for pieces of 9mm or less, the kiln can be turned off.

However, you need to think carefully about firing quickly.  When realistically will you be able or actually need to take the piece out of the kiln?  If it is the next day or after work, then a slower firing reduces the risks of rapid firing and still enables you to take the piece out when needed.

Wednesday, 13 May 2015

Frit Making

Start with clean cullet. Rinse or wash off all the dust and felt tip marks. Spread out to dry, or put the collection in the kiln at about 200C to dry.

An example of clear glass cullet

It is possible to start with the dry glass, or you can heat the glass to a temperature in the 300C to 450C range in a stainless steel container.  Then take the hot container with heat resistant gloves and pour the hot glass into a bucket of water (do not pour the water onto the glass, as a great deal of steam will be produced burning you).  The fractured glass can then be further broken down in size after being dried.

Smashing the glass can be done in a number of ways.  A small amount can be made in a mortar and pestle.  Larger amounts can be put between sheets of newspaper, in plastic bags or any other container that will keep the glass from shooting all over.  Hit the glass package with a hammer multiple times.

You can build a frit maker from pipes with end caps.  Fit one pipe inside the other with the glass between the ends and pound the glass between the two surfaces.  There are a number of variations on this method of production.

An example of  a commercially available frit maker

Large scale frit production can use coffee grinders, or adapted waste disposal units. Waste disposal units tend to produce a lot of frit of the same size, while coffee grinders with blades produce frit sizes related to the time the glass is ground.

Example of a suitable coffee grinder

Example of a garbage disposal unit

When you have created a pile of frit, sieve it into various sizes with screens.  It is best to discard the fine frit and powder, as they contain contaminants that are difficult to clean out and will discolour the finished product.

Example of a commercially available set of seives

You also need to ensure the metal is removed from the glass by using a strong magnet.  Fridge magnets will not do.  Put the magnet in a plastic bag and run it over the frit several times.  When finished take the bagged magnet to the waste bin and remove the magnet.  The metal will fall into the bin.

Example of  a pair of strong magnets

It is a good idea to rinse the remaining frit to remove dusts and ensure the frit is clean.  Again you can put the frit on a metal tray in the kiln to dry.  Put the frit into a closed container to keep it clean until needed for use.

Wednesday, 6 May 2015

Fibre Board Moulds

There are a lot of moulds available in a variety of shapes and sizes through various suppliers.  But sometimes you want a simple or special shape that is not available to buy.  Fibre boards provide you with the material to make your own special moulds without great expense.

The ceramic fibre boards tend to come in a variety of thicknesses, mostly about 1 metre square.  Boards in 10, 15 and 25 mm thicknesses are commonly available. 

You should work in a well-ventilated area using a dust mask to avoid inhaling the dust and fibres.  See Gregorie's Glass for information on safety.

You can cut a wide variety of shapes into fibre board with just a craft knife.  You can smooth the shapes with just sandpaper. The shape can be smoothed with sandpapers in both natural and hardened states.  If a lot of material is taken off hardened shapes, it is advisable to coat that area with hardener and cure it again to ensure maximum durability.

A question that will arise is whether to harden or not.  This depends on the durability you require.  A board that is not hardened does not require kiln wash when fired.  However, as it is soft it is easy to break.  A hardened fibre board mould always requires kiln wash or other separator.  It does become durable and almost rings when tapped once it is hardened and cured.  If the shape needs to be preserved for further use, hardening is advisable.

It is also possible to stack the boards to make deeper forms.  The boards should be stacked and pined together with copper of high temperature wire such as kanthal to keep them from moving both while shaping and in use.

Fibre boards are relatively inexpensive, in comparison with commercial moulds.  They are not as durable as some, but provide a means of obtaining special shapes unique to your work.

Wednesday, 29 April 2015

Large Bubbles
Some times you want large bubbles, but when you don't, you need to know about the causes of, and ways to prevent, bubbles.

Causes and prevention of most large bubbles relate to volume control, layup, rate of advance and top temperature.

Volume control. We all know that glass tends toward being 6-7mm thick at full fuse temperatures. Any less volume and the glass thickens at the edges, so trapping air under the glass which will push up and through given enough time and temperature.

Layup. The lack of volume control is compounded by layups which do not allow the air to escape from under the edges of the piece.

Rate of advance. These two problems are compounded by asap, or even just rapid, increases in temperature. The glass softens quickly and the air finds it easier to push the glass up than to escape from under the edges. Slowing down is part of the answer.

Top temperature. A high top temperature softens the glass to the extent that any lack in volume control, layups that have hollows or traps for air, and rapid increases in temperature all allow the expanding air (and there will always be some) underneath the glass to push up and often through the glass.

These factors reinforce the Low and Slow mantra.

Other factors can promote bubbles, although the ones above are the most common. Debris between the glass and the shelf, or between glass layers can cause bubbles, given the right conditions. Small shallow indentations in the shelf can be the source of bubbles from underneath the glass. This can be identified by observing where the bubbles occur in relation to the shelf.

There are some things that can be done to reduce the likelihood of bubbles:

Wednesday, 22 April 2015

Mixing COE

Our use of Coe as an equivalent for compatibility can lead to difficulties. The only compatibility that can be relied on is that given by the manufacturer. No manufacturer can attest to the compatibility of another manufacturer's glass. They can only verify their own.

So, if you mix manufacturers' glass even though advertised as the same COE, it does not make them compatible. There is much more than expansion rates that goes into compatibility. You need to test different manufacturers' glass against each other before you use it.

These are notes on aspects of compatibility.

Wednesday, 15 April 2015

Dams for Melts

There are a number of commercial moulds, dams and rings to contain pot and screen melts.

You can, of course, make your own. A simple one is to use 10 or 15 mm fibre board to contain your pot or screen melts. Cut the size and shape of hole you want into the board and that will contain the glass.

You can place this directly onto the kiln washed shelf. No fibre paper is absolutely required unless you want to. You can weight the board by placing the supports for the screen or post directly onto the board.

If you want to use the board more than once, you need to harden it with colloidal silica and fire it. Then you always need to put a separator on it at each firing to ensure it does not stick to the glass melt.

This process allows you to make custom shapes and sizes without great expense. With a bit of ingenuity, you can provide your own textured bottom to the melt.

Wednesday, 8 April 2015

Safe Cooling Speeds

Almost everywhere you read that you should not open your kiln to cool it. But we know that the kiln cools progressively slower as it nears room temperatures. So how do we safely cool the kiln at rates the glass can cope with.

Some advocate directing a fan at the closed kiln on the grounds that air movement will cool the kiln faster than still air will. Yes, but at a negligible rate due to the insulation properties of the the kiln.

It is much safer to program a sensible cooling rate all the way to room temperature from 370C. This allows you to open any viewing ports knowing that any rapid cooling will be counteracted by the controller switching on the heat. Upon hearing that power input, you can close the ports partially until the kiln cools a bit more.

At lower temperatures, you can open the lid or door. The controller will operate as before warning you of too rapid cooling, so you can reduce the opening until more air is required.

Wednesday, 1 April 2015

Clay Moulds

Making your own clay moulds is not as difficult as often imagined. You do need to have access to a kiln that can fire to bisque temperatures, as they are above the usual limitations of glass kilns.

There are a variety of clays you can use – paper clay, school clay or any cheap clay from a ceramics supplier. Make sure any “clay” you buy from craft shops is in fact ceramic clay and not one of the no-fire or oven fire “clays”.

The method for slab moulds is fairly simple. You need to roll out a sheet of even thickness. To do this there are a few items required.
  • Board to support the rolling of the clay
  • Rolling pin of sufficient length
  • Cloth for the bottom of the clay
  • Battens of 8-10 mm thick

Place the smoothed cloth on the board.
Place or fix the battens to the width needed.
Put the clay onto the cloth and work it out to the approximate thickness you require by hand.

Use the rolling pin to complete the process by pushing the clay in front of the rotating pin to get an even thickness. This will require a number of passes, both away from and toward you. You can patch any shallow places by simply putting a small piece of clay there and going over it with the rolling pin.

At this point you can impress any design you like on the wet clay, or you can incise the leather-hard clay later.

If you are shaping the clay, you can use a variety of materials to hold the clay in the shape you want. You can use another form to support the whole of the clay if desired.

Set the clay aside to dry, ideally where air can get to all sides/surfaces. The clay will shrink during this drying process. It will take longer to dry if the clay can dry from only one surface, and it is more likely to crack. So if only one surface can be exposed to the air, you should cover the whole piece with a damp cloth to slow the evaporation from the top surface.

When the clay is leather hard, you can sand or mark the clay. Then have the clay fired to bisque temperatures. When you get the fired form back, you should sand any roughness away, as this will be the bottom surface of your completed piece.

It is important that you kiln wash or provide other separators between the mould and the glass, as the clay and glass will stick together. Because the clay and glass expand and contract at different rates, the glass will be broken or crazed by the ceramic.

Wednesday, 25 March 2015

Straight Edges on Thick Pieces

As glass tends towards 7mm at full fuse, it is difficult to keep straight edges on thick pieces as the glass moves. If you want straight edges without dams, there are a few solutions:

1) Don't flat-fuse - apply less heat work so that the stack stays vertical instead of spreading. The degree of tack fuse required will be a subject of observation and experimentation.

2) Plan on trimming the edges straight. You can use a saw or grinder and then either cold work the edges to polish, or fire polish.

3) Add a couple of centimetres or so on each side of your base glass, so that a 20x20cm piece becomes 24x24cm, and flat-fuse as normal. The volume change will (mostly) be absorbed by the extra glass, so that you can simply trim it back to the right size and cold work the cut edges.

Wednesday, 18 March 2015

Straight Lines on Thicker Pieces

When you use a partial layer on two or three layers, you will get wavy lines as the thicker glass spreads more than the thinner. E.g., if you have 3 layers, especially near the edges, the fully fused glass will spread out, while the two layer areas will keep their volume. If you have straight lines at the edge, they will no longer be straight at fusing temperatures.

To keep the lines straight on thick pieces, build the design upside down, so that the final top design is down to the shelf. The shelf side is the coolest part of the glass being fired, so it will distort less. In addition, the weight of the glass above will assist in keeping the design elements in place.

This is often referred to as “flip and fire” 

Wednesday, 11 March 2015

Flow Melts without Metal

“Flows” seem to be popular just now.  These are variations either on aperture drops using multiple holes or on screen melts depending on the number of apertures.  This is not a complete tutorial, just some notes on how to prepare a more sound and so more lasting project.  It is not complete, as these flows are essentially incomplete screen melts or aperture drops.  The techniques and methods that apply to them apply to melts also.

It is inevitable that steel of a thickness that can support the weight of inch thick glass when encased will break the glass sooner or later.  Why take the risk of incorporating a material which is not of a size or nature suitable for inclusion in glass?

Instead, one alternative is to use fibre board.  Take a piece of 10mm board (or thicker if desired) and cut holes on a grid pattern.  This is done so that the whole board can be supported on steel rods for safety.  If you are using thicker board, you can insert the steel rods into the fibre board, so ensuring they cannot come into contact with the glass, but still support the whole structure.  I have done 300mm square fibre board drops without support, although the top surface is significantly bowed at the conclusion.  Because you have quite a bit of weight on the supporting board, I would include at least a couple of 3mm stainless steel rods (kiln washed)  at 1/3 and 2/3 distance across the piece, so plan your holes with that in mind.

Another alternative is to kiln wash - or coat in bead release - stainless steel rods of 3 or 4mm diameter and make the grid from them.  This grid can be supported on a rectangular frame of dams or a drop out mould.  As you are going to relatively high temperatures, bead release may work best in this situation. Lay the glass on top of the rods.

Fire your glass with the appropriate rates.  Remember in annealing that you are dealing with a piece that has pretty large differences in thickness.  You therefore need to lengthen the annealing soak, and slow the annealing cool.

Once the piece is cool, you can take it out and put it upside down.  Slide the steel rods out and gently remove the fibre board from between the “legs” of the piece.  If you have used a grid of steel rods, they can removed from the glass, by gently pulling as you twist the rods.  You may have to soak the rods in water to help soften the bead release.

Now you have a flow with no inclusions and much more likely to last. 

Wednesday, 4 March 2015

Texture Moulds

Texture moulds are popular but expensive and with limited designs. You can make your own unique ones with only a few items of equipment. 

An example made by someone else


Various forms of clay can be used. Roll out a flat piece using boards of 8-10mm thick to support the rolling pin and give consistent thickness to the clay. The board underneath should be covered in paper or cloth to make an easy release. I have found that grease-proof paper as used in baking works very well.  It releases easily from the clay.  

Paper clay provides light weight moulds that do not hold a lot of heat, but any standard clay will do the job. There are two approaches to developing the pattern. You can stamp the pattern into the wet clay with any prepared design on a stamp or other textured material. The other is to dry the clay to leather hardness. You can then incise the pattern you desire directly into the clay. Fire to bisque temperatures, and sand to remove any rough areas or undercuts. Kiln washing the mould before use is essential.

Using a patterned roller to impress the design on the clay

Fibre board

Various fibre boards can be used. Ceramic fibre board, Kaiser Lee board, Vermiculite board, insulating ceiling tiles such as Armstrong, and calcium silicate boards can be incised and marked as desired. The advantage to the ceramic fibre, insulating ceiling tiles, and Kaiser Lee boards is that they allow air to pass through the material. Kaiser Lee board of these three provides the easiest surface for incising. Calcium silicate has no fibres, but requires a separator. Ceramic fibre and Armstrong ceiling tiles have fibres, requiring a bit more work to get a smooth surface. Armstrong tiles require a separator, but ceramic fibre boards do not unless you harden them for durability.

Fibre paper

Three millimetre fibre paper gives a easy material for cutting with craft knives or scissors to the design wanted. You can draw through an existing stencil or copy the design with carbon paper. It is not easy to produce designs with lots of detail.  It is quick, does not require separators, but is probably a single use material, unless you use mould hardener and then kiln wash, although it still will be delicate. For large projects, the paper should be fired first to ensure the combustion of the binders do not produce gasses to cause bubbles or fogging.

Loose Material

Sand, whiting, and kiln wash provide easy materials for one-off designs. You can quickly draw the design you want into the flat loose material with your fingers, or tools. You can also use found items to press into the loose material. Place the glass gently on top of the material and fire. If you use sand, you should dust it with kiln wash or aluminium hydrate to ensure the sand does not stick to the glass.

Unique Designs

All of these methods will provide unique designs which will distinguish your work from others.

Wednesday, 25 February 2015

Calcium Silicate for Moulds

I have recently discovered calcium silicate board. It is light weight, with no fibrous material. It is workable with wood carving tools and can be smoothed with sanding papers. It is manufactured in 25 mm thick boards, and so is suitable for shallow forms.

It requires no hardeners, but it does require kiln wash to ensure clean separation of the fired glass. It also is durable, but needs careful storage and handling.  It does not appear to warp with repeated heating.

High performance calcium silicate board will withstand temperatures of 870C for limited periods, according to the manufacturers.  It is best to support the shelf well and consider it a mould or shelf for temperatures at or below full fusing where it is known to perform very well.

An additonal factor in its favour is that it is significantly cheaper than either ceramic fibre board or vermiculite board.

Wednesday, 18 February 2015

Flattening stringer

Placing stringer is often difficult. Not simply to put it into place, but to keep it there. People tend to hold the pieces with glue. However, the glue burns off before the stringer is anywhere near even tack fusing temperature. This allows the stringer to roll. Also an excess of glue will boil off during the heating and so move the stringer even more than gravity will. Two methods are effective in reducing the ability of the stringer to roll, by flattening one side.

One method used by Bob Letherbarrow is to hold a stringer that is much longer than needed against the glass grinder bit. Use your thumb to hold the stringer against the bit. Slowly pull the stringer between the bit and your thumb or finger, using light pressure, to hold the stringer against the spinning grinder bit. This will slightly flatten one side of the stringer.

Another method is to place the stringers on a prepared kiln shelf, making sure they do not touch. Take the temperature quickly up to a tack fuse, soak for a few minutes and turn off. This will take only an hour or so and enables you to prepare a number of stringers with a minimum of effort.

In both cases, cut the stringer to the length needed and place on the glass with the flat side against the glass. Glue it very lightly to hold it in place while moving the piece to the kiln if it is absolutely necessary. When the glue burns off the stringer should not roll around. It is best, of course and if at all possible to place the stringers in the kiln, so no glue is needed. 

Wednesday, 11 February 2015

Equalisation temperature

In my view a schedule has the following stages.
  • Initial rate of advance to bubble squeeze,

  • Rapid increase in temperature to target, or working temperature,

  • Quick fall to temperature equalisation (often called the annealing point),

  • Slow decrease in temperature - to keep internal stresses at a minimum - to 110C below that temperature equalisation point,

  • Faster cool to 100C or less.

Of course, some of my firings have up to 10 segments, so don't mistake the stages as equivalent to schedule segments. The following graph is a generalised version of these stages.  The times and temperatures are for illustration only.

The equalisation temperature is what is most often called the annealing point. This is a mathematically determined temperature at which the glass most quickly anneals - has stress relieved. However, the way kiln formers work, annealing does not occur at one temperature point on the controller output, because of the inherent inaccuracy of our kilns and controllers. The soak at the annealing point has the purpose to equalise the temperature throughout the glass before proceeding to the anneal cool.

There is little point in soaking above this temperature, only to have another, lower temperature soak at the published annealing point. The soak at the annealing temperature will negate any effect of a soak at a higher temperature. So, a soak above the annealing temperature will simply slow the whole cooling process.

Of course, the soak at the equalisation temperature must be long enough to get the whole substance of the glass to the same temperature. The thickness of the glass will determine the length of this equalisation soak. Fortunately Bullseye have published a table to help determine the time required.

The slow decrease in temperature is to keep all the substance of the glass to within 5C difference on the cooling. Thus, the rate of cooling is related to the thickness of the glass. It will be increasingly slower with increasing thickness. The cooling to around 110C below the equalisation temperature is all part of the annealing process. The more rapid cooling after that is to control the rate of temperature fall to avoid thermal shock.

Wednesday, 4 February 2015

Thermal Shock

Thermal shock is a term for a break caused by a too rapid change of temperature within a piece of glass.

"Glass tends to be 
1) very brittle, 
2) expand and contract quickly when subjected to temperature changes, and 
3) is an insulator (when solid) and therefore does not readily conduct heat. 
That is why glass is highly susceptible to thermal shock"

This can occur on both an increase or decrease in temperature. Glass conducts heat poorly.  The ideal is to keep the temperature differentials within the glass to 5C or less.  This is the purpose of the anneal cool.  The risk of thermal shock can be increased by different thicknesses across the piece. Greater care is required in cooling these pieces than those of uniform thickness.

A piece showing large differences in thickness and so at greater risk of shock


The break normally is straight through the glass without following the edges of the various pieces of glass.

This shows the break crossing multiple colours of glass

The line of the break will be rounded if it parted on the heat up. In some cases, the glass will have stuck back together if it was dammed or the break was gentle enough to avoid pushing the glass apart.

If the shock occurred on the cool down, the edges will be sharp.  

The edges will also be sharp in a slump whether the break occurred on the advance or the reduction in temperature.  If the pieces fit together perfectly the break is likely to be in the down phase.  If the pieces are slightly different shapes the break likely occurred in the rise in temperature phase.

Other kinds of breaks are possible and are described elsewhere.

Wednesday, 28 January 2015

Glasses at Risk of Compatibility Shift

Many people take their fusing glasses beyond the tested parameters of the manufacturers in pot and screen melts and combing and casting operations. It has been speculated that there are compatibility shifts of hot colours and of opalescents.

Reading, and some experience, lead me to the belief that is the colouring minerals that are the key to which glass will shift in compatibility. Colours made with sulphur and selenium are more likely to opalise and also change their compatibility at extended times at high temperatures. Extended time is in the region of an hour or more. High temperatures are those over 850ÂșC

The colours at most risk of compatibility shift seem to be:
and a few bright and olive greens, but not dark greens.

Of course testing, using polarising light filters, is required to determine which will remain compatible after long, high temperature firings.  A method of testing is given here.

High temperature compatibility shifts are discussed here.

Wednesday, 21 January 2015

Compatibility Shift at Higher Temperatures

People experience breakages of their pot and screen melts that do not seem to have anything to do with annealing or glass sticking to the shelf. The common suggestion is that there has been a compatibility shift of the glass. This view is re-enforced by the opalisation of the transparent hot colours experienced by most.

Bullseye indicates in their glass notes that some colours are not suitable for high temperature work. This probably applies to other fusing glasses too. My experience leads me to believe that this compatibility shift occurs with all the opalescent glass colours as well as the hot ones. Further work will appear soon. is required to determine if there are any general indicators of the kinds of glass that are likely to develop incompatibility at high temperatures.

If you are concerned about the lack of durability of your piece due to possible incompatibility, you need to include tests with the firing. To make this test, place a piece of each colour used in the melt on a double layer of clear. If you are using a single base piece, ensure you leave space between the colours. It is best to place each colour on its own stack of clear. Also place a stack of clear glass as thick as your blank along side the other test pieces. Put all those pieces somewhere within the kiln out of the way of the area the melt will occupy and fire the lot together.

When cool, take all the pieces from the kiln and check the test pieces for compatibility. Do this check with a polarising filter to determine whether there is any incompatibility by looking for the halo showing the degrees of incompatibility.

If any or all, of the the pieces show stress, check the clear stack for stress. If the clear also shows stress, the annealing has been inadequate, rather than just the compatibility shift. Ideally, this process should be conducted in every firing.

Performing these tests will give you confidence in the durability of your piece, as it will show the levels of stress in the finished piece.

Wednesday, 14 January 2015

Annealing Unknown Glass

Sometimes you may want to use a glass in kiln forming when its characteristics are not known, such as for bottle slumping. It is possible to determine the approximate annealing point of this glass in your own studio. This tip on compatibility testing gives you the information to do the test.

If you do not want to go to that detailed effort for a one-off process, you can adopt the shotgun annealing approach. This does require some observation of the glass, of course.

You need to observe when the glass has reached the temperature for the process you are performing. This will enable you to compare the behaviour of this unknown glass with what you normally use. This will give some idea of the relative annealing temperature to use. If a higher temperature is required for this glass than your normal glass, a higher annealing point can be assumed. The difference in top temperature can be added to the annealing point of your known glass.  If the top temperature is lower, you subtract the difference from the known glass' annealing point.

Set the annealing temperature to be 10C to 20C above the predicted annealing temperature and soak there for 30 to 60 minutes. This will help ensure the glass is all at the same temperature throughout. Set the annealing cool to be at about 30C per hour for pieces up to 6mm for the first 55C. The next segment should be about twice that to 110C below your chosen annealing temperature. The final segment can be around 150C per hour to 100C.  For thicker glass, the annealing cool should be proportionately slower.

This may seem an excessive, overly cautious process, but as you get to know the characteristics of the glass, you will be able to alter the schedule. This is a conservative and safe process to ensure your glass is well annealed.  And to be certain, you should check the cooled glass with polarised light filters.

Wednesday, 7 January 2015

CoE Varies with Temperature

Information from Bullseye shows that the Coeficient of Linear Expansion changes rapidly around the annealing range.

The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F -­ 572F).................90.6
300C-400C (572F - ­752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0

Bullseye glass is probably typical of soda lime glasses designed for fusing.

This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.

In addition, here is an illustration of the effect. 

(If the owner of this illustration comes across this, please let me know, as I have lost the source)


The object of plating is to modify the original colour, either by changing the tone or the intensity. This will, for example, darken a piece of glass where it would otherwise be to bright; or it will modify the colour to better blend with the surrounding pieces.

A further use of plating is in conservation, where the additional detail is placed on a separate piece of glass and placed in front or back of the original.

In leading, you normally use high heart lead. This is lead with a heart of 7mm or 10mm instead of the usual 5mm. Other heights are available, of course. The 7mm heart will accommodate two 3mm pieces, but if you are using thick hand made glass, you may require the 10mm high heart.

Comparing the Arrangement
Try the glass combination with each piece on top. Often there is a difference in tone or texture. Choose the one that suits your composition best.

Before finally fixing the glass together, make sure they are very clean as there will be no opportunity to clean the inside again. Try to avoid finger prints on the insides while you do further work with the glass.

Make sure the glass fits the cartoon lines. You will be sealing the two pieces of glass together, so there is no opportunity to change the shape later. There are a variety of traditional methods of sealing the glass, but the easiest modern approach is to copper foil the edges to ensure that no cement creeps between the pieces.

You then fit the glass into the came as for thiner pieces. Where you have a combination of heart heights, you can simply slip the ends of the lower heart cames inside the leaves of the high heart leads. The differences in height are small enough that no special support is needed for the thinner glass unless you feel better with the single layers of glass supported above the work surface.

Saturday, 3 January 2015

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the strain points. The higher one is the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 45C above and below the annealing point. The ideal point to anneal would be at the annealing temperature, as annealing occurs most rapidly at this temperature.

However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. So a soak at the annealing temperature is required. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to the 515° point during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before the annealing begins.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken. But no further annealing will take place. If you do not anneal properly the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

The glass is brittle below the upper strain point temperature, although it is less and less likely to be subject to thermal shock as it nears that point.  It is after the upper strain point that you can advance the temperature as fast as you like without shocking the glass.  So, if you have a glass that gives its annealing temperature as 5125C, you can safely advance the temperature quickly after 560C (being 45C above the annealing point).