Texture Moulds

 Texture moulds are a form of bas relief in reverse. The texture of the mould is the bas relief. The glass formed over the shapes is in negative relief. The light is refracted through th
e back to give an image of bas relief although the surface is smooth.

 

Example of wave form texture mould

 These moulds are prone to produce bubbles at the generally recommended tack fuse temperatures. The glass often sticks to the mould if there is not sufficient draft to the parts of the image, or if insufficient separator is used. Often the moulds are produced with a rim around the edges, which trap the glass.

 The usual temperatures are too high. These moulds are an exercise in patience. The temptation is to fire higher than slumping temperatures to get good definition in the glass. However, a number of problems, especially bubbles, can be avoided by staying at the high end of slumping temperatures. This means the top temperature would be about 680C. To compensate for this low temperature, the soak needs to be three hours or more. To be sure the definition desired has been achieved, peeking near the end of this long soak is necessary. 

 Moulds that are produced with a rim around the edges can trap air and create bubbles. The rim forms a perimeter dam to confine the glass. If the rate of rise is quick to a high temperature, the edges can be sealed against the rim before all the air has escaped. It is advisable to cut the glass for these rimmed moulds a bit smaller than the internal dimensions formed by the rim.

 

Example of textured area surrounded by a rim

 Use of single layers on texture moulds can lead to large, thin bubbles. This is most prevalent when using high temperatures. Since the single layers tend to form more slowly than an already fused two-layer piece, the temptation is to use higher temperatures. The higher temperatures soften the glass to such an extent that often bubbles form over the lower areas of the mould. Instead, low temperatures with extremely long soaks should be used to allow the glass to conform to the undulations of the texture without dog boning or developing bubbles. Of course, peeking will be required to determine when the texture is achieved. With single layers, the surface will have greater undulations than with two layers. The thinness of the single layer cannot fill the depression the way two layers can.

 

 Rapid rates to high temperatures can produce internal bubbles too. These moulds have a multiplicity of hollows and depressions. Just as people are warned about depressions in their shelves, the depressions in the texture moulds can cause bubbles too. This means there are even more possibilities for bubble creation than on apparently flat shelves. Long slow bubble squeezes are required to allow air from under the glass.

 Glass sometimes sticks to the mould. This is most often blamed on insufficient separator. Boron nitride is a good separator for these moulds especially if you go to tack fusing temperatures. At slumping temperatures, kiln wash will normally be sufficient. Both of these separators need to be applied carefully, as there are relatively steep slopes throughout the mould. Spraying needs to be done from at least four angles to ensure all the sides are covered.

 Painting on kiln wash is a little more difficult, as the solution is so liquid, it tends to run down the slopes without much sticking. One means of rectifying this is to tip the mould in a circular motion to move the still liquid kiln wash solution around the slopes.

 Less often thought about is the draft of the shapes of the mould. If the slopes (draft) in the mould are too steep, the glass will “grab” the ceramic mould, because the ceramic contracts less than the glass when cooling. If shapes of the mould are steep and deep enough, the glass may even break as a result of this compression of the mould.

 

An example of some nearly vertical elements and a rim

 Of course, if a flat front surface is required, a higher than slumping temperature must be used. This is required to allow the glass to flow to the lower portions of the mould. It still should be as low as possible, but with very long soaks.

 Avoidance of bubbles on, and sticking to, texture moulds is best achieved by avoiding high temperatures, use long soaks, use two layers, and avoid extending glass to the rim. These combined with observation of the progress of the firing will produce the best results.

 

Other information is available:

Low Temperature Kilnforming, an Evidence-Based Approach to Scheduling, an ebook

Bas relief

Layups promoting bubbles

 

Annealing Requirements for Shaped Pieces.

 Experiments related to slumping show that shaped items such as slumped, textured and kiln carved glass need annealing for at least one layer thicker than they are. The annealing for one layer thicker than the calculated thickness provides the most stress-free result for the finished product. Annealing for the calculated thickness does not always produce a stress-free result.  

Full Fuse

 This indicates that an evenly thick 6mm thick piece will get the best result from an anneal as for 9mm.

Texture Moulds

 A piece of glass on a texture mould with 3mm or more differences in height requires careful annealing. The more defined/sharper the texture, the greater care will be required. A 6mm blank on a mould with 3mm variation taken to a well-defined texture needs to be annealed as though it were 18mm thick. A sharp tack requires annealing as for a piece 2.5 times its actual thickness plus another 3mm.  This gives the 18mm/0.75” thickness annealing requirement for the 6mm thick piece.

Kiln Carvings

 The same kind of calculation applies to kiln carved items as for sharply textured pieces. Pieces with less sharply defined profiles can be treated as one of the more common tack fused profiles.

Credit: Vitreus-art.co.uk

Tack Fusings

 A rounded tack fused piece of a 6mm base with 3mm tack elements that is being slumped will need annealing as for 21mm.  Twice the actual thickness plus 3mm giving the annealing requirement as for 21mm/0.827”.

 A contour tack of the same dimensions as given in the first example will require annealing as for 19mm/0.75”. The annealing requirement when slumping is for 1.5 times the thickness plus another 3mm.

In General

 The general approach to annealing shaped pieces is to calculate the thickness for the anneal and add one layer more to get a good anneal for slumped and other formed pieces. 

 The research and the reasoning behind this approach is given in LowTemperature Kilnforming, An Evidenced-Based Guide to Scheduling available from the Etsy shop VerrierStudio and from Bullseye. 

Texture Moulds and Glass Sizes

I had an overhang [on a texture mould] and I heard a pop and opened kiln and saw it cracked off the mold. … [The piece] is 2 layers Bullseye irid placed face down and Tekta [on top]; the mold was sprayed 3-4 times with zyp and Thinfire; and I put mold on kiln posts. [I] fired to 1440[F].

Diagnosis 

The overhang of the glass caused the break. As the glass heats it expands. The ceramic does not expand as much as the glass.  This means even more glass will hang over the edge than at the start.  As the glass reaches slumping temperatures, it begins to drape over the edge. At the same time the glass on the interior is beginning to slump into the textures.  When the glass has fully taken up the texture, the overhanging glass will be touching the outer sides of the mould. This means at the end of the heating and soaking part of the firing, you have the ceramic mould partially and tightly encased in glass.  The glass has formed around the ceramic.

Credit: theavenuestainedglass.com

The physics of the two materials are that glass expands more than ceramic. On cooling, the glass grips the sides of the ceramic mould tightly. This is because it shrinks more than the ceramic.  In this case, the ceramic was stronger than the glass and the strain caused the glass to break.  Upon occasion the opposite can happen.

Two other notes.

The temperature of 781°C is higher than needed.  You will need to do a bit of experimentation to find the right combination of temperature and time for each mould.  You could consider 630°C as an initial temperature with a 90-minute soak.  Bob Leatherbarrow (p.161) describes a method of scheduling a sequence of slightly higher temperatures with soaks.  If the texture is not forming (as determined by observations), you can advance to the next segment with a higher temperature and see how that goes.  When the appropriate amount of texture has been achieved, advance to the cooling to anneal segment.

Iridised surfaces provide a very good separator.  With the iridisation down against the mould, it may be unnecessary to use Thinfire, especially when you already have used boron nitride as the separator.

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

Incomplete definition on texture moulds

An enquiry on incomplete definition in the glass from texture moulds:

I have this texture mould, but I’m not getting much definition. I’m using a single 3mm layer of 96. Do I need to go hotter for longer?

155C - 750C, 15 minutes

Full - 516C, 140 minutes

49C - 371C, 0 minutes

Full - 49C, off

My response:

You have sensibly increased the temperature at a moderately slow rate (for 3mm).  This ensures the glass is evenly heated through by the time it reaches the working temperature.  It is slow enough for you to be confident that most of the air has been squeezed out.

If you wanted to be more cautious about bubbles,  you could introduce a slow increase in temperature - maybe 50°C - from 600°C to 650°C.  You may want to soak there for 30 minutes, although it may not be necessary.  Once that segment is finished you can resume at 75°C to the top temperature.

I would not increase the temperature as you are already at risk of dog-boning the glass.  I would extend the soak time to 180 minutes at 750°C. You need to check frequently after the top temperature is achieved.  A quick peek is all that is required to see if the texture is fully reflected in the top surface.  You may find success by using a lower temperature, say 730°C, but it will require at least an hour more soak time.

The piece above conformed completely to the 12mm depth of the mould with a soak of three hours at 750°C. There was incomplete formation of another test piece at 740°C for three hours. So the 10°C made enough difference for complete formation over this depth.  With less extreme heights, a lower temperature or a shorter soak would be possible.

Once the texture is assured, you need to advance to the next segment.  Or, if it is not achieved by 10 minutes before the end, extend the soak.  Check your controller manual on how to skip to the next segment, or to extend the soak.

As an aside, your annealing soak and cool is very long and slow for 3mm.  You can regain the time used in the slow ramp rate and soak. Review the requirements for a single sheet of glass.  A 60-minute annealing soak and a cool rate of 83°C/hr. to 370°C is an adequately slow anneal cool for a 3mm piece.

You may find more success with a 6mm sheet.  The weight of glass helps it conform to the texture mould.  I have found that a slow ramp rate (about 150°C) to the strain point of ca. 540°C, followed by half that rate to top temperature allows a reduction in soak time to achieve the required definition. This reduction in soak time can be one half hour less than the time required to get good definition on a 3mm sheet.

The strategy outlined here for the scheduling is using the principle of slow and low and long firings.  It is much easier to control the results of the firing by using moderate ramp rates to lower temperatures combined with longer soaks and periodic peeking to check on progress.

If you do not have the time to devote to peeking when the schedule is at the top temperature, you should investigate the method of programming a delay to the start of the firing.  Your controller manual will give the method of using this function.

Texture moulds work well with the slow and low principle of kilnforming.  Long soaks may be required with periodic observation to determine when the process is complete.

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

Texture moulds

"I could use some help here please. I’ve tried this sun mould 3x and as you can see all 3x I get a hole.  If you could tell me what I’ve done wrong I would greatly appreciate. They were all full fused to 1430F (776C)."

Example of the problem

There are a range of views that have been given on how to make texture moulds work without the glass developing bubbles.

closer view of one example

These are a summary of the suggestions made to the enquirer.

Not enough glass thickness. The view is that glass needs to be 6mm thick to be used on texture moulds, as the viscosity of glass tends to draw glass to that thickness, robbing from other areas making them thin and prone to bubbles.

Glass always wants to go to 6mm.  Not always.  It depends on temperature.  The kiln forming temperatures we use results in a viscosity that tends to equalise the forces at 6 – 7 mm.  Hotter glass will flow out more thinly, until at about 1200C, the glass is 1mm or less thick.

Full fuse two sheets first.  The object is to avoid placing two separate sheets on top of the mould, creating the potential for more bubbles between the sheets, as they may slump into the mould at different rates.

Too hot. As the glass increases in temperature the viscosity is reduced and can no longer resist the air pressure underneath the glass.

Use a lower temperature. The idea is to keep the glass relatively stiff to resist bubble formation.

Bubble squeeze needed to avoid trapped air.  Another way to reduce the amount of air under the glass is to allow the glass to relax slowly at a temperature below which the glass becomes sticky.

Elevate the mould.  The idea is that hot air circulating under the mould will help equalise the temperature of the mould and the glass.

Drill holes at low points. This gives air escape routes under the mould, assuming the mould is slightly elevated.

Go lower and slower.  Use a slower rate of advance toward a lower top temperature with longer soaks to avoid reducing the viscosity, but still get the impression from the mould.

Now for a different viewpoint.

None of the views given above are wrong, but they all (except in one case) fail to consider the fundamentals of obtaining texture from such a mould.

It is apparent that the temperature used was too high because the glass had low enough viscosity to allow the air underneath to blow the bubble.  The suggestions of thicker glass, bubble squeezes, lower temperatures, drilling holes and elevation of the mould are ways of reducing the amount of air or resisting the air pressure.  They are not wrong, but miss the fundamental point.

That fundamental point is that you need to raise the temperature slowly on these texture moulds to allow the glass to fully heat throughout. By doing this most of the air has a chance to filter out from under the glass before it conforms to the edges of the mould.  It is simpler to use the slow advance rather than a quick one with a slow-down for a bubble squeeze.  The glass is more certain to be the same temperature throughout by using a slow rate of advance.  Glass with an even temperature can conform more easily to the undulations and textures of the mould.

Mostly, the recommendations given are to use two layers, or 6mm of glass that has already been fused together.  This gives greater resistance to bubble formation and reduces the dogboning and needling of the edges.

However, you can form in these moulds with single layers.  There are of course certain conditions:

• You must advance the temperature slowly.  A rate of 100C per hour will be fast enough.
• You can add a bubble squeeze soak of 30 minutes at about 630C as additional assurance of removing most of the air.  The bubble squeeze is done at a lower temperature than usual, as the glass is less viscous because the slow rate of advance has put more heat work into the glass.
• The top temperature should not go beyond 720C. Beyond that temperature the viscosity of the glass drops quickly and so becomes subject to bubble formation.

The soak at the forming temperature will need to be long and observation will be needed to determine when the glass has fully conformed to the mould. Quick peeks at intervals will show when the design is visible on the top of the glass. The time will vary by:

• Mould texture complexity 
• Type of glass (opalescent or transparent),
• Heat forming characteristics of the glass,
• Viscosity of the glass or colour,
• Etc. 

Be knowledgeable about how to extend the soak or to advance to the next segment of the schedule to take advantage of your observations.

Your observation may show that you can do the texture formation at a lower temperature in future. This will provide results with less separator pickup and better conformation to the mould without excessive marking. 

You will need a long soak in either circumstance. This will be in terms of hours not minutes.  If you do these texture moulds at slumping temperatures, you will probably need at least twice your normal soak.

You can do a lot to fool the single layer glass into doing what you want by using low temperatures and long soaks. See Bob Leatherbarrows's book on Firing Schedules.  He gives a lot of information on how to manipulate glass through heat work - the combination of temperature and time.  You might also consider obtaining my book - Low Temperature Kilnforming.

Most of the search for the right temperature, fails to note that the important element is how you get to the temperature. You can get the same result at different temperatures by using different rates of advance.

Kilnforming is more than temperature, it is also about time and the rate of getting to the temperature. By concentrating on temperature, we miss out on controlling the speed and the soak times. You can do so much more to control the behaviour of the glass at slow rates, significantly long soaks, and low temperatures.

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.

Textured Side

There is a little concern about whether the textured side of the glass pieces in leaded and copper foiled glass should be towards the inside or outside.

The traditional advice is to have the textured side toward the inside.  This is based on the piece being used as a window. It is easier to keep the weather side clean if the smooth side is on the outside. The same thinking leads to the recommendation to allow the cemented panel to rest with the smooth (outside) down.  This minimises the thickness of the putty and so allows less water to collect on the outside horizontal leads.

If the window is not primary glazing, it does not matter which side, nor how consistent you are in placing the glass.  It becomes a matter of aesthetics – which ever way you prefer is fine if it gives you the effect you want.

There is a small visual effect if you are using transparent glass.  There is slightly more dispersion of light if the textured side is outwards. 

Placing the textured side inwards can be useful if you wish to indicate a rough surface contrasting with a smoother one.

These considerations show that the placing of the textured side is largely determined by the function of the panel and the aesthetics applied.

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

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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.

www.amaco.com

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.

bluewillowstudio.net

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.

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

Clay

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.

Layering Glass Textures

When using textured glass there is a decision to be made on whether the smooth or textured side is up.

Oddly, the largest, but thin bubbles occur when putting the smooth sides together. it seems that as the glass is not perfectly flat, it holds air within the fused piece.

The fewest bubbles seem to be promoted by placing the rough side down on all pieces. This is easy as cutting is done on the smooth side anyway, and so no reverse cutting is required. It seems that the rough side of the glass provides ways for the air to escape during the bubble squeeze although it does promote micro bubbles within the glass.

If more bubbles are desired, you can place the textured sides together. That seems to allow the majority of the air out, but still leaves the micro bubbles from both sheets.

I have had good results following the Bullseye recommendation to keep the smooth side up on all layers. 

Achieving a Matte Finish by Cold Working

Although sandblasting and then firing a piece can achieve a matte finish, there are several other ways to improve the quality of the final finish.

One of these involves the use of manual sanding after sandblasting in order to smooth out uneven spots and achieve a better final finish.

• Start with a 400 mesh diamond hand pad. It shouldn't be necessary to start out with a lower mesh (coarser) pad.

• Alternatively use wet/dry silicon carbide sandpaper. A combination of 400 mesh paper, followed by 600 mesh paper will work well.

• If you're using sandpaper, place a sponge between the paper and your hand for improved comfort and to improve the evenness of the final finish.

• An alternative to hand sanding is to use a electric sander or grinder, but be careful with the pressure you use, as it is possible to grind into the surface with a rapidly spinning surface. You also need to keep the surface wet to avoid heat build-ups.



You can also use a lathe with appropriately shaped wheels to give decorative effects to the object.

Firing for a Matte Finish

Glass can be fired to take on a satin appearance that is both appealing to the eye and pleasing to touch.

The first step toward the matte finish is to sandblast the piece after fusing, then fire to a temperature between 600C and 675C. A short soak - or no soak at all - is all that is needed.

The exact temperature needed depends on a number of factors, including:

• The specific glass being used. A soft glass such as black generally needs to be fired to a lower temperature than glasses that do not absorb the heat so easily. Every colour and type of glass will behave a bit differently, so experimentation and record keeping is critical.

• The grit and type of sandblasting medium. Generally, a grit from 120 to 200 is preferred, with aluminium oxide performing a bit better than silicon carbide – which can often lead toward some devitrification.

• The particular kiln being used. Your kiln is a bit different from any other one. Start with a temperature in the middle of the 600-650C range and adjust depending on the results you achieve.

• The finish you want will vary with only a few degrees difference. This means that you have to observe the firing. Make sure you keep good records of the specific firing schedule used so that you can make adjustments if needed for future firings.



Some variations can provide distinctive elements to the finished piece.

• Masking certain elements before sandblasting can provide contrasts of texture within the piece.

• Firing at a lower temperature for longer can give the results you want, without any additional marking on the bottom of the piece.

• To keep the matte texture, any subsequent slumping of the piece should be done at as low a temperature as possible.