Rakes for Combing

It is of course, possible to buy commercial tools for combing hot glass.  But with a little ingenuity, you can make your own for a small amount of money and some effort.

My raking tool is a metre long round stainless steel rod, 8mm in diameter. I sharpened it on a  grinder for metal rather than my glass grinder. Then I bent a right angle to give me 75mm "hook". The handle is a piece of broom handle. I drilled an 8mm hole in the wood and hammered it on.

A longer metal and shorter wooden handle works better than the one I made with a long wooden handle, as there is no wood near enough the heat to burn. If you do have a long handle,  soak it in water to keep it from burning.

It is possible to make a rake using mild steel rod, but it is more likely to spall and drop flakes into the glass.  Both metals need to be kept cool.  Rest the rake in a bucket of water before the first pass at combing.  As the glass stiffens and you need to wait for the glass to come back to a combing temperature, put he rake back into water to cool it.  If you try to comb with a hot rake, it will stick to the glass.

It is important to have a handle made of an insulating material to avoid any possible electrical shocks.  It also makes for a more comfortable handle that does not heat up.

Safety gear is required to protect eyes and clothing from the heat.  It is not possible to have the kiln open at around 900C without it.  This is the face, hand and arm protection I use.  The coated visor protects your eyes against the infrared radiation from the kiln.  The gloves can be the aluminised silver colour ones or the kevlar ones.  The alminised ones are easier to manipulate things with.  The arm protectors are aluminised too. They are easy to put on and give additional safety to the body.

Natural fibres should be worn to avoid clothing bursting into fire.  I use a denim jacket reversed for additional chest protection.

Fibre Paper and Fibre Blanket

Refractory fibre is generally divided into paper and blanket.  There is a distinction between the two relating to binders and thickness.

Papers are those which have binders to keep the pressed fibres in a sheet or roll.  These binders burn off during the firing process

Blanket does not have binders and is much thicker.

These two forms of refractory fibre generally exhibit a difference in thickness.  

Papers range from cartridge paper thickness, to around 6mm thick.  

Blanket tends to be 25mm and thicker, although there is some 12mm I understand.

Firing Uneven Layers

There are a number of people firing stacked layers of glass in a pyramidical fashion to melt the layers down, in a nod to the 1950’s.

Annealing of a full fused platter of this nature is easier than a tack fused one. The degree of contour still in evidence will be important in determining what the annealing schedule should be.

Full or Tack Fuse

In this example shown by Vicki Urbich there are at least four layers - five if the base is two layers, although in this case there is only one.  A full fuse at 800°C will not be enough to give a flat piece for a trivet or platter.  You could extend the soak at 800°C but, it is better to go to 816°C for ten minutes rather than an extended soak at lower temperatures to avoid devitrification.

Damming

The edges of this piece will be wavy, unless dammed, because of the uneven layering.  Placing dams around will give crisp edges to the piece, even though the stacked pieces will round and spread. 

Annealing stacked pieces

Anneal this set-up for at least 9mm for a full fused piece. The pieces will spread and attempt to fill the gaps between the stacks.  Even with an 816°C fuse, the pieces will not be perfectly even in thickness.  To be safer, and account for the remaining unevenness, anneal as though it were 12mm thick.  Other lay-ups will have slightly different requirements.

If it is to be tack fused, you will need to anneal considering the different thicknesses across the piece.  You will have nearly 12mm thick at the thickest and only 3mm at the thinnest.  The generally accepted recommendations are to anneal for twice the thickest part - 24mm in this case.

The anneal is more than the length of soak. It is a combination of the soak and the rate at which you cool to at least 370°C. The cooling rate is directly related to the length of the soak.  If you require twice the length of soak at the temperature equalisation soak, you will require half the speed of anneal cool.  The Bullseye Chart forAnnealing Thick Slabs will give you the relevant rates regardless of the glass you are using. The temperature points will change for other glasses, of course, but the rates remain the same.

Rate of Advance

The earlier problem this lay-up gives you is the heat up to avoid thermal shock. 

The heat up of 4 layers of glass stacked on a single or even two-layer layer base is more difficult than for even layers across the whole piece. Each upper piece shades the heat from the lower ones, making for cool and hot areas next to each other.  With four layers, each layer needs to heat through to transfer its heat to the one below.  This means the bottom of the stack will take a long time to become as hot as the top layer.  Meanwhile, the uncovered glass is getting as hot as the top of the stack.  This often leads to the bottom layers breaking from the stress of the uneven heating.  

Graham Stone suggests 100°C (180°F) per hour for four, even layers. As this is four uneven layers, the rate of advance should be at least half that. This should be used all the way up to the bottom of the bubble squeeze to allow all the glass to heat at the same rate. Glass generally reacts better to a slow, steady contant rate of advance in heat, than faster rates with multiple soaks.

Bubble Squeeze

The bubble squeeze for this single layer base piece can be as quick as 50°C per hour over the 50°C range.  It does not need to be slower, as the weight of the stacks pushes the air out between layers more quickly than large, even and lighter layers.  A double layer base requires a slower bubble squeeze because the weight of the stacks will push the air out to be between the two base layers.  This means a rate of 30°C or even 25°C through the range.

Then you can go faster to the top temperature.

Firing uneven layers requires extreme care on the initial heat up to avoid thermal shock.  A high fusing temperature is needed to get an even thickness across the piece.  Annealing is easier to calculate for even pieces, but must be much more cautious for tack fused items.

More detailed information is available in the e-book: Low Temperature Kilnforming.

Drying Kiln Washed Moulds

There seems to be a popular notion that newly kiln washed moulds must be cured before use.  I'm not sure where the information comes from, and no reasoning is given.  It is suggested that that quickly heating newly kiln washed moulds to 550°F (290°C) is important.

If you want to make sure the mould is dry, this may not be the best way to do it.  All ceramics have a quartz inversion at around 225°C.  This a very rapid increase in volume of 2.5% that often leads to cracks and breaks in ceramics when the rate of advance is quick.  The mould will react better and last longer if the rate of advance is slow until that inversion temperature is passed.

This is a reason to advance the temperature slowly when slumping or draping with a ceramic mould.  Another reason to heat slowly is to avoid steam formation within the ceramic body.  If the steam is created over a short time, the force can be great enough to break the ceramic.  To ensure the water evaporates, a soak at 95°C for a significant amount of time is a better, safer option.

But in addition to all these precautions, it simply is not necessary to cure kiln wash on slumping and draping moulds made of ceramics.  The glass does not begin to move until after 540°C (about 1000°F). Therefore, the kiln wash will be dry long before the glass gets near slumping temperatures.  Any vapor caused by evaporating water will escape through the vent holes in the mould or under the glass at the rim, as it will not form a seal until higher temperatures.

newly kiln washed mould beside others already fired

If you want to be sure your kiln wash is dry before you put the mould in the kiln, you can leave it in a warm ventilated space, or even on top of your kiln while it is being fired.  Using either drying method will dry the kiln wash sufficiently before the glass is placed on the slumping mould.

Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary, step in kiln forming.  There are exceptionally good reasons to avoid rapid firing of damp moulds. The exceptions can occur with texture moulds and those intended for casting that do not have vents.

Large Bubbles

As you move up from smaller pieces to pieces that occupy most of the shelf, you sometimes begin to get large rounded bubbles at tack fuse and burst ones at full fuse.

Image from B Stiverson

You have to go back to basics to discover the cause.

Schedule

It is not likely to be the schedule. It has worked for smaller items. But it is important to review the schedule.  Is it like others you have seen? Is it similar to what the glass manufacturer recommends?  Both these will reassure you that the schedule is OK, if not perfect, or to revise it.

Cleanliness

Going back to the basics relates to the cleanliness of your kiln, among other things.  Even a small speck of material under the glass can result in a bubble. Although the grit lifts the glass off the shelf only a fraction, as it heats up the glass slumps around that and creates an air pocket.  That grows as the glass heats up and creates a large diameter bubble. If there is no grit in evidence, you need to check another element of your kilnforming practice.

Shelf

The large bubble might often occur in the same relative place in the kiln, although different places on the glass pieces, depending where they are placed.  This is an indication that you may have a hollow in the shelf. It may not have been obvious with smaller pieces.  You need to check the shelf with a straight edge. If any light is seen between shelf and edge, you have a depression in the shelf.  It may only be a sliver of light, but that indicates a depression which is enough to create a large bubble. That must be fixed.

Image from Suze

There are temporary and permanent fixes for avoiding bubbles due to depressions in the shelf. 

The temporary fix is to use 1mm fibre paper on the shelf, to allow air out from under the glass.  This can be topped with Thinfire or Papyros. Alternatively, a thin layer of powdered kiln wash can be smoothed over the fibre paper to give the smoothest back possible in the circumstances. You can use a plasterer’s float, or simply a piece of float glass.

The permanent fix is to sand the shelf smooth and level.  A method for doing this is here.

Single Layer Bases

If you are firing with single layer bases, there may be nothing wrong with the shelf.  It is typical in tack fusing to use single layers with glass placed decoratively around the surface of the base.  This leaves gaps where the base glass is exposed.  Even though the whole piece may survive the differential heat up of the exposed base glass and the covered parts, there is the possibility of creating an air pocket under the exposed base.  This comes from the weight of the stacked glass pressing any air out to the side.  If the design is unable to provide a route out for the air, the possibility of creating an air bubble increases.

It is possible to create conditions to reduce the possibility of these large bubbles developing. 

One solution is to use a layer of fibre paper as for a shelf with slight depressions.  This allows air out from under the glass, even with a single layer layup.

The other solution is to change the rate and temperature of the firing.  By using the low and slow principle, you can reduce the risk of bubbles.  Use a much slower rate of advance to a lower temperature with a longer soak you can achieve the look you want without bubbles.  This utilises the concept of heat work.  It does require observation to determine when the effect you desire is achieved and then advance to the next segment.

Further information is available in the ebook Low Temperature Kiln Forming.

Oxidisation of Foil

Often, life intervenes between foiling and soldering.  This frequently results in the foil not accepting the solder very well, because of the mild tarnish that has occurred in the meantime.

It is a good idea to clean the foil of any possible corrosion before soldering when there has been an interval between the two processes. It is enough to clean foil with a mild abrasive such as a foam-backed scrubber from the dish washing, or fine steel wool.  I prefer the scrubber as it does not introduce another metal.  

Some people prefer a vinegar and salt solution to apply to the tarnished foil.  I am concerned about the introduction of an acid into the process causing further problems later.  I don't recommend this method of cleaning.

I then coat the exposed foil with a film of paste flux to protect from further tarnish. This acts better than any loose covering of cloth or plastic to protect from oxidation.  The purpose of flux is to both provide a "wetting" agent for the foil to accept the solder,  and to prevent oxidisation. Liquid flux cannot provide protection, once dry,  from the copper tarnishing.  I prefer the use of paste flux to reduce boiling of flux and to keep the copper free from corrosion.  The paste flux will not indefinitely prevent oxidisation, but will do so for a week or two.

Pin holes in melts

Pin holes in screen and pot melts are the result of very small bubbles rising to the surface.  These bubbles are sometimes within the glass melted, but more often come from small amounts of air trapped within the flowing glass.  These are perceived to be unsightly, or make it impractical to make a functional piece from the melt.

There are ways to minimise bubble formation or to deal with the formed bubbles.

Bubble Formation

In pot and screen melts, the glass spirals as it touches down onto the shelf. This spiralling action can trap small amounts of air as each successive spiral forms beside the previous one. Efforts at prevention of tiny bubbles in the final piece need to concentrate on this fact.

Bubble Squeeze

A preliminary element in bubble prevention is to have a long bubble squeeze to allow the glass to settle in the pot or on the screen so that the rest of the process can proceed with a minimum amount of air trapped within the flowing glass.

Two-Stage Drop

In some cases. it is possible to have the glass flow from the pot onto an angled shelf where the spiralling glass has to flow from the initial touch down to the edge and then flow onto the shelf.  This allows any tiny bubbles initially trapped to escape before the final drop onto the shelf.  This provides two mixing processes and means that a lot of clear glass needs to be included to avoid a complete mix of the colours.  It requires careful selection of the original colours to avoid a brown or black result.  It also requires a big kiln with sufficient height for a two stage drop.

This two-stage drop is of course, not suitable for a screen melt where you wish the glass strands to remain.  Nor is it suitable when you wish to have many “pools” of colour mix in the final piece.

Where the two-stage drop is not practical or suitable other methods can be used.  These relate to scheduling, cold working the surface and re-firing the piece.

Schedules

Scheduling relates to using a soak at full fuse temperature before proceeding to the anneal.  The melt will occur at 850°C to 950°C.  You can cool as fast as possible to a full fuse temperature of about 810°C and soak there for an hour or more.  This allows the small bubbles to surface, break and heal.  Schedule the rapid cool to the annealing soak, once the high temperature soak is complete.  This will eliminate lots of the bubbles, but not all.

A sample friring schedule from bubble squeeze upwards and then down to a high temperature bubble reduction soak

Cold Work

Cold working the melt is about abrading the surface to open the bubbles that are just emerging to form a small dome at the surface.  Sand blasting is a common form, as usually kiln wash or fibre needs to be removed from the bottom of the melt, and some devitrification from the surface.  It would be possible to continue to grind the surface of the glass to eliminate the small depression in the glass caused by the now opened bubble, but this is likely to expose more bubbles that were at a slightly deeper level. What next?

Fire Polish

As you will need to do a fire polish firing after blasting or grinding the surface, you can use a full fuse temperature to allow the surface to become plastic enough to fill the bubble holes.  Remember to schedule the firing as though the piece were at least 12mm thick.  You may find that more bubbles are exposed in addition to the ones healed at the conclusion of this second firing.

Flip and Fire

An alternative is to fire upside down.  You will have noted that there are no bubbles on the bottom of the melt.  This is because the bubbles have risen through the heated glass.  This physical fact can be used in the second firing.  Fire with the melt surface to the shelf.  It is best to have a clean and newly kiln washed shelf, or fibre paper (not Thinfire or Papyros) under the glass. Fire the glass to a full fuse or high temperature tack fuse with a significant length of soak to allow the bubbles near the original surface to move toward the interior of the glass.  After firing, the glass will need thorough cleaning before being fire polished. This should leave you with a pin hole free piece.

Conclusion

Achieving a pin hole free pot or screen melt requires several stages of coldworking and firing.  This makes melts inexpensive in materials (it is scrap of course) but expensive in time and firings.

Strong Frames for Stained Glass Panels

Metals

Zinc is a popular material for framing copper foiled or leaded glass panels.  It is stronger than lead – up to eight times.  It gives a feeling solidity to the edges of the panel. 

However, it does have some disadvantages.  It is difficult to patina evenly and obtain the same colour as patinaed solder.  It resistance to progressive corrosion is weaker than lead. It requires special tools to fit around curves, making it best for rectangular panels.  It does need a saw to cut evenly, but so do a lot of the stronger metals.  A look at other options is worthwhile.

The strongest option is stainless steel.  This is difficult to cut and has special welding requirements, so is only useful in large and high corrosion installations.

Mild steel is widely available and cheap.  In certain circumstances – mainly small, thin profiles – it can be soldered.  The most secure joining is done with welding.  This requires equipment that stained-glass workers do not usually have.  However, there are a large number of metal workers that can to the work for you.

Brass is more expensive than mild steel.  It is an alloy of copper and tin and so can be soldered with the tools we normally use.  It is about half the strength of stainless steel, but three times the strength of zinc.  The tin content leads to a better patina result than zinc.

Copper is up to twice the strength of zinc, but the price fluctuates more than zinc.  It can be soldered. It requires different patina solutions than used for solder.

Aluminium is the same strength as zinc, but requires different joining methods as aluminium welding is a specialist activity.  Still, it will work on rectangular items with screws at overlapping joints.

More information on the relative strengths of various metals is given in a post on metal strengths.

Strengthening lead came

Lead is weaker than lead but can be bent to conform to curves and indentations for irregular perimeters.  If copper wire is incorporated and attached to the foiled glass, the soldering of the lead came to the joints at the intersections of the solder lines and the coper/came combination will provide greater strength than the zinc alone. 

When wanting to strengthen the perimeter of rectangular or shaped perimeter leaded panels, you can use 10mm “H” lead came soldered as usual to the whole piece as an alternative to soldering the wire to the panel.  Run the copper wire in the open edge of the “H”.  Pull the wire tight at the bottom and sweat solder at each corner.  Run the wire to the top on each side, where you can make a loop for attaching hanging wires and sweat solder the wires there too.  Then close the two leaves of the lead with a fid until they come together forming a single straight line.  If you want, a “U” or “C” edging came can be soldred to the outer edge of the "H" came to cover the line created by folding the leaves.

This post gives more detail about the process of incorporating copper into the perimeter of a leaded panel.

Pot Melt Saucers as Dams for Melts

Preparation

Many ceramic plant pot saucers can be used as circular moulds.  Most are unglazed and will accept kiln wash easily.  Some are unglazed, but polished to such an extent they are no longer porous.  These and glazed flower pot saucers need some preparation before applying kiln wash.

Plant pot with saucer

Polished and glazed saucers require roughing to provide a key for the kiln wash solution to settle into.  This can be done with normal wood working sand papers.  You may want to wear a dust mask during this process, but not a lot of dust is created.  You could also use wet and dry sandpaper or diamond handpads with some water to reduce the dust further.

If the sanding of the surface does not allow the kiln wash to adhere to the saucer, you can heat it.  Soak it at about 125C for 15 minutes before removing it from the kiln to get the heat distributed throughout the ceramic body.  One advantage to the ceramic is that it holds the heat, because of its mass, for longer than steel.  Apply kiln wash with a brush or spray it onto the warm saucer.  As it dries, apply another layer of kiln wash.  Two or three applications should be enough to completely cover the surface.  If not, then you probably will need to heat up again before repeating the process.

Alternatives to plant pot saucers

There are alternatives to the saucer approach to getting thick circles from a pot melt.

 

Fibre paper

You can cut a circle from fibre paper and melt into that.  The advantage of fibre paper is that it requires little preparation other than cutting and fixing.  You may have only 3mm fibre paper and want a 9mm thick disc.  Simply fix the required number of layers together with the circle cut from each square.  The fixing can be as simple as sewing pins, copper wire, or high temperature wire.  Then place some kiln furniture on top of the surrounding fibre paper to keep it in place on the shelf during the melt.  This furniture can often be the supports for the melt.

Fibre board

If you find cutting multiple circles of the same size a nuisance, you can use fibre board.  Simply cut the circle from the board with a craft knife.  You will probably want to line the circle with fibre paper, as the cut edge of fibre board can be rough.  Alternatively, you can lightly sand the edge.  Wear a dust mask and do this outside, if possible, to keep the irritating fibres away from the studio. If you want a thicker melt than one layer of board can give, just add another in the same way as for fibre paper.

In both these cases, you may wish to put down a layer of 1mm fibre paper to ensure the glass does not stick to the shelf and does not require sandblasting.  

The advantage of the fibre paper or board alternative to flower pot saucers is that you do not need to kiln wash anything unless you want to. If you do not harden the fibre paper or board, it will not stick to the glass.

Vermiculite board

Another alternative is vermiculite board.  The advantage of this is that it comes in 25 and 50 mm thicknesses, so you can make the melt as thick as you like without having to add layers.  You can cut the vermiculite board with wood working tools.  Knives will not be strong enough to cut through the vermiculite board. You will need to kiln wash or line the vermiculite with fibre paper, as the board will stick to the glass without a separator.

Damless circles

Of course, if you want a circle without concern over the thickness, you can do the melt without any dams. You need to ensure that the shelf is level.  Any supports for the pot will need to be both kiln washed and far away enough that the moving glass does not touch the supports and distort the circle.  In general, one kilogramme of glass will give a 300mm circle, so your supports need to be further apart than the calculated diameter of the circle.  An undammed circle will vary from 6mm at the edge to as much as 12mm at the centre, depending on temperatures and lengths of soaks.

First Firing of your New Kiln

First Firing of your New Kiln

I have just been reviewing information on kiln elements. I have discovered the reason you need to do your first firing with the kiln empty of everything. No kiln wash, no kiln furniture, nothing. Vacuum the kiln to take out any dusts from travel.

The element forms a protective layer of aluminum oxide during the first firing. If there are elements of kiln wash, dusts, or glass, this will inhibit the ability of the oxide coating to be uniform. The uniform coating of the elements is important to the long life of your elements. There are other things of course, but this is the initial, essential element of preparing you kiln for use.

After this first firing you can add the other elements of kiln wash, furniture, and even glass.

In summary, fire your kiln clean and bare. No kiln wash, no furniture.

Sam Smith adds: [This] applies to kilns made with Kanthal A1 elements. Those are the good ones which last pretty much forever. Cheaper quality kilns can have nichrome elements which do not develop the coating. The firing the kiln empty allows the oxide coating to form. If you do a firing where combustion takes place such as firing fibre paper or shelf paper you should realize those combustion products are attacking your element coatings and it may be worth while venting the kiln and or firing the kiln up empty after the firing in order to protect or allow the development of a new layer of coating covering the kanthal. Kiln wash us is cheaper and safer for the long term life of your kiln elements. Smart people only purchase kilns with Kanthal elements.

Bubble Squeeze for Multiple Layers

Difficulties often occur with bubble formation within pieces composed of several layers. There are a couple of factors in addition to the number of layers that have an influence - temperature and rate of advance to the bubble squeeze temperature.

Temperature

The top temperature for the bubble squeeze does not need to change with multiple layers. It is the advance to the bubble squeeze that needs to change in relation to the number of layers.

Rate of Advance
It would  be suitable to reduce the rate of advance to about three-quarters of the two-layer schedule to account for three layers.  And a reduction to one half of the two-layer schedule for a four-layer piece would be appropriate. The reasons for these slower rates of advance follow.

A normal rate of advance for two even layers would be about 200°C per hour to the bubble squeeze temperature.  Sometimes a very slow rate of advance is used from 50°C below the top of the bubble squeeze.  This strategy can continue to be used for thicker pieces made up of many layers with some modifications.

Multiple Layers

But for a three-layer piece, slowing the rate of advance to about 150°C is important to assist in a good bubble squeeze.  This helps get all the glass at the same temperature by the time the bubble squeeze is approached. Glass is a good insulator, and also a poor conductor of heat. This slower advance allows the bottom layer to be at the same temperature as the top piece.

For a four-layer piece, a rate of about 100°C would be suitable.  When the lower point of the bubble squeeze is reached (about 50C below the upper soak), the slow rate of advance can be used to go to the upper end of the squeeze, using the normal soak length.  

This illustrates that the more layers of glass in the stack, the slower the rate of rise must be in the bubble squeeze range.

Five Layers and Beyond

For pieces made up of more than four layers, a different strategy is needed to ensure the heat reaches the bottom layer of glass.  Graham Stone* calls this the “catch-up” schedule. It is essentially an overnight schedule with temperature equalisation soaks of 20 minutes at 125°C intervals all the way to the bubble squeeze. At each stage the rate is increased by 10°C.

This means that with a first segment rise of 20°C per hour, the second from 125°C to 250°C is at 30°C with a 20 minute soak, then 40°C to 375°C soak for 20 minutes, 50°C to 500°C and soak for 20 minutes, and finally 60°C to 625 for a final 20 minutes with 70°C to your normal bubble squeeze temperature.  This will take about 17 hours before you go on to the forming temperature.

This long heat up schedule illustrates the problem of getting the heat to the bottom layers of the stack, and the need to squeeze the air from between the layers.

Thicker pieces apply more weight to press out bubbles from lower layers, but only if the lower layers are equally as hot as the top.  This requires long schedules.

An alternative approach to this bubble squeeze problem is to fuse two layer pieces of the appropriate number to achieve the thickness required.  If these are fired with good bubble squeezes there will be a minimum of bubbles.  Combining these 6mm blanks will give fewer bubbles with a proper bubble squeeze.

Another approach is to start with 6mm glass as it comes from the maker.  This is not always possible, because it is not common for 6mm fusing glass to be made in anything but clear.  It may be possible to incorporate the clear within the stack, if it is not appropriate on the bottom.  These thicker sheets have fewer bubbles proportionally than 2mm or 3mm sheets.  So there are fewer bubbles in the final piece.

Of course, placing shards of glass at the corners, or sprinkling a very thin even layer of powder between multiple sheets will also help reduce bubbles between layers, but it is the slow rate of advance to the bubble squeeze that is the important element.

*Firing Schedules for Glass; the Kiln Companion, by Graham Stone, 2000. ISBN 0-646-39733-8

Further information is available in the ebook: Low Temperature Kiln Forming.

Texture Mould Firings

Texture moulds are essentially permanent kiln carving moulds.  These are moulds that use different levels within the mould to develop the imagery by giving different thicknesses to the glass. Temporary or single use moulds can be made from fibre paper, although not with the same subtlety as the ceramic ones.

Single layers

Many people wish to use a single layer in these texture moulds. For a single layer, a tack fuse is a high as you can take the temperature. This will not give you the definition that you could get with higher temperatures unless you use very long soak times. 

Good definition

To use higher temperatures, you need at least two layers (6mm) to avoid distortion, dog boning, possible bubbles and needling.  With two layers you can go to full fuse temperatures to get the best conformation to the mould.

Low temperature firings

You can get better definition at lower temperatures by going slowly to your target temperature. This slow rise in temperature – ca. 100C - or less - per hour – all the way to the lower end of the tack fuse range – ca. 730C to 750C -  needs to be combined with a long soak, possibly two or more hours.  This long soak allows the glass to sink into the depressions of the mould without shrinking, dog boning or needling.  This shows that the speed you use has a major effect on the target temperature.

Separators

Another element of difficulty in the use of these moulds is the separator used. Kiln wash is adequate, especially if you are using the lower temperatures.  Boron nitride is a popular choice for those going to higher temperatures.  Using iridescent glass with the coated side down to the mould provides an additional separator, allowing higher temperatures to be used.  It can enhance the appearance of the piece too. 

The way you schedule for texture moulds is an interaction between the thickness of the glass, the rate of advance, the target temperature and the kind of separators used. With these four factors interacting, the choices are not simple.

More detailed information is available in the e-book: Low Temperature Kilnforming.