Breaks in Slumping

Despite not opening the kiln until it was 120 degrees, a ¾" crack formed in the middle of a 15" x 15" flat fused 1/4” slumped platter, fused, then slumped. The bottom layer was four squares, topped by a single layer of Tekta. How can I avoid crack at the meeting of the squares?

The fuse schedule was

• 300°F to 1110°F, 30’

• 200°F to 1240°F, 60’

• 600°F to 1475°F, 17’

• 9999 to 900°F, 60’

• 150°F to 700°F, 1‘

The slump schedule was

• 250°F to 1225°F, 60’

• 9999 to 900°F, 90’

• 100°F to 700°F, 1’.

Any ideas why the platter cracked? Thanks.

Hannah Gulick adds: “We see this phenomenon happen a lot with this kind of checker board pattern. I find it occurs more on the underside than the topside, but it's always at the point where the four corners are meeting. It's a perfect storm situation to do with the design, colors, mold, etc. Bullseye recommends doing checker board patterns in general at a slower initial rate of heat for slumping, usually 100°F/hour.”

This piece shows extreme stress in several places.

Stress generally is most concentrated at corners and points in any piece. It is logical to treat pieces with meeting corners and points more cautiously than normal. This will include a stress test of the full fused piece to determine what stress there may be at that point in the process. I would have annealed as for 9mm, resulting in an annealing schedule of:

• 1.5 hour anneal soak

• 125°F – 800°F, 0’

• 225°F to 700°F, 0’

• 600°F to room temperature

If the stress test shows significant stress, it needs to be fired again and annealed for at least one more layer thick, depending on the amount of stress showing, and checked again before slumping.

I have found that slumping a full fused piece needs to be fired for at least one layer thicker than usual. In this case, where known stress points are at corners, even if one rather than multiple points, more caution is needed. I would anneal as for two layers thicker, or at least for12mm, resulting in:

• anneal for 2 hours

• 100°F to 800°F, 0’

• 180°F to 700°F, 0’

• 330°F to room temperature

Colour Density for Dropouts

Recommendations for a transparent sky blue apart from sky blue for a drop out so I need high saturation?

Image credit: The Glass Hub

Keep in mind that the colours wash out more at the shoulder of a drop out as it thins. To obtain a more even colour throughout the length of the drop the colour should graduate from the most dense at the shoulder to paler at the centre of the drop.

One way to achieve this is to use more layers of the same colour toward the shoulder. Another is to use a combination of overlapping shades of the same colour family with the most dense at the shoulder.

This might be achieved by cutting overlapping rings of colour, or crescents of the colours arranged in circles on the blank. These will give a banding of colour rather than a gradation in the final piece.

Anomalous Annealing Schedules

Why do schedules vary so widely?

I frequently observe strange annealing schedules promoted on the internet. These include:

• extremely long anneal times with rapid cools risking thermal shock.

• single slow cool rates, using more time and electricity than necessary

  • to 371°C/700°F.  There are better cool arrangements that fit the needs of cooling glass.

  • to room temperature.  Again using more time and electricity than needed.

  • somewhere between.  These still do not adhere to the requirements of the glass.

• Cool rates with staged soaks. There is no use for soaks below the strain point (about 470°C/800°F) as any annealing stress is locked in by that time.

• Cool rates unrelated anneal soak lengths.  Adequate cool rates are directly (but inversely – longer soak = slower cool rates) related to the length of the anneal soak.

• Cool rates ending at 371°C/700°F which are inadequate for thick glass.

They are strange, because there is a well researched document giving annealing times and cooling rates published by Bullseye for Fahrenheit and Celsius, 

It may be understandable that users of fusing glasses other than Bullseye are cautious about the use of this table.  But it is a well researched table supported by independent academic research, which is applicable to all soda lime glass. [McLellan and Shand (1984), Glass Engineering Handbook, 3rd Edition, New York, McGraw Hill]  Only the annealing temperature needs to be changed, as all the rates and times apply equally to all fusing glasses, and even float glass.

Yes, sometimes changes are required for:

• Profile

  • Contour fuse needs times and rates for 1.5 times the thickest part

  • Tack fuse needs times and rates for 2 times the thickest part

  • Sharp tack or sinter needs times and rates for 2.5 times the thickest part

• Contrasting colours, and differing viscosities need times and rates for at least 3mm thicker than the profile indicates

• Extreme forming such as drops and deep slumps also need times and rates for at least 3mm thicker than the profile indicates

These matters are explained in greater detail in my e-book Annealing Concepts, Principle and Practice available from Bullseye and Etsy

Stabilising stringers with powder

Credit: Creative Glass Guild

Can stringers be stabilised without glue?

To stabilise a single or multiple stringers a few grains of powder are all that is required to move from the workbench to the kiln. Such small amounts of powder will not show up even at low temperature tack fusing.  This method will avoid using glue to keep any light weight item in place - frit, shards, small round pieces, etc.

Of course, if the work is being transported over distances between studios, some form of glue will be required.

But too many people reach for a glue as the first rather than last option.

To see another method of stabilising stringers requiring higher temperatures see this post.

Playing in the Sandbox

How can sand pictures be produced in glass?

This process provides flowing, abstract images that can be used as autonomous pieces or formed into other objects, such as free drops, bowls, cut for jewellery or into pattern bars.  The appearance provided is unique to this combination of using frit and pressing.

In principle, this process is the same as creating sand pictures.  The process is in three stages: making the box, adding frit, and pressing.

The Sandbox

Determine the size of the box.  It should not be more than two-thirds the size of your kiln shelf depending on thickness.  Thicker glass pressed to 6mm will spread more than thinner.  As a guide, 12mm should have an allowance to spread to about 1.3 times the original size; 19mm should have an allowance to spread about 1.5 times the original dimensions.

Cut two sheets of the same size from clear fusing glass. One will be the front. The other will be the back.

Determine whether the image you are creating will be portrait, landscape, or square.  Orient the sheets in the appropriate way to have the top away from you.  Choose the top piece of the pair and cut two 6mm strips from the designated top.  This gives you a lip to be able to pour the frit into the box easily.

Box formed with bottom and sides glued to back and front.  The filling lip shows on the right.

From another piece of clear glass cut two 6mm strips for the sides.  If you cut them the same length as the side of the glass, they will stick above the back about 3mm. You can cut this off, but it really is not a worry for the construction of the box.  These strips form the spacers to allow the frit to be poured into the box.  Their thickness will determine the amount of frit needed to fill the box.

Get out the back sheet and clean and prepare it for attaching the strips. My preferred method is to glue the bottom 6mm strip on its edge with super glue.  It is advisable to wear plastic gloves when gluing the strips, to avoid sticking your fingers to the glass.  Super glue cures quickly and does not delay the construction of the box.  It burns out cleanly without any health and safety concerns.  Place a thin film of super glue on one edge of the strip.  Attach it to the bottom by placing it carefully at the edge of the sheet.  Do the same for the sides.

  

When the strips are stuck down to the back, place  a thin line of super glue on the top edge of the strips in preparation for attaching the top sheet.  Using a strip of wood placed at the bottom of the backing glass will help in placing the sheet accurately. Lower the sheet from contact with the bottom to the strips forming the sides of the box.

When the glue is cured, inspect the sides of the box for gaps. If there are gaps, use clear Sellotape to seal the gaps in the sides. It will burn off cleanly in the kiln.

Adding the Frit.

Place the box on an easel or other support so it is slightly tipped backwards.  This helps ensure the box does not fall toward you while working on it.  It also allows the frit to slide toward the bottom rather than bouncing off the other frit.

The early stages of filling with the box on a stand

The size of frit you choose to use will affect the final appearance.

·        Generally, powder will appear greyer and more opaque than frit. This is due to the multiplicity of tiny bubbles between the grains of powder.

·        Fine and medium frit provide more clarity than powder.

·        Coarse frit provides the most clarity, but with fizzy bubbles between pieces of frit.

When preparing to place the frit in the box, it is a good idea to take small amounts out of jars and place it into small cups to avoid contamination of the main source of the frit.

Pouring the frit into the sandbox

You can use a jeweller’s scoop or a teaspoon to move the frit from the cup to the box.  Tip the frit into the box above where you want the colour to be placed.  

Using a wire to poke frit through to lower layers.  This also shows  a difference between front and back.

If the frit does not land just where you want it, you can move it with stiff wire that is long enough to reach the bottom of the box.  Gently sweep the frit with the end of the wire toward the place you want the coloured frit to be.

Using a jewellers scoop to add the frit.

Continue adding colours to create the profile and shapes you wish.

You can make additional alterations to the way the frit is placed.  You can poke the frit from one layer into lower layers with a stiff wire by pushing the wire directly downward.  You cannot do this more than 2 or 3 centimetres deep, as the frits and powders become compacted.

A thick copper wire being used to poke down from an upper layer to the lower ones.

When filled to the top or to your desired level, use the fourth strip to close the box.  If full, glue the strip to the top.  If not full, cut strip to the length needed to drop into the opening of the box.  Place a couple of drops of super glue on the top of the already placed strip to keep it in place while moving to the kiln.

The Pressing

Prepare the shelves

You will need two shelves for each pressing. One is the base to hold the glass and the spacers.  The other is to provide the weight to press the glass thinner.

Clean off old kiln wash from the shelves. Experience shows that adding new kiln wash over old for this process promotes the sticking of the kiln wash to the glass.  Add new kiln wash that performs well at extended times at upper temperatures.  I find Bullseye shelf primer works very well.

Once partially dried, with the pink beginning to pale, you can smooth the surface brush marks.  Some use balled up material such as tights to rub over the surface.  I find very good results from rubbing lightly over the kiln washed surface with a sheet of paper between the palm of my hand and the shelf.  The advantage of doing this smoothing while slightly damp is that no dust is created that needs to be cleaned away.  The disadvantage is that too much pressure will pull bits of kiln wash from the shelf.

Do not use fibre papers as the separator.  The glass will be moving within the space between the shelves.  It will pick up and incorporate parts of the fibre paper, if used.

If you have shelves of different thicknesses, reserve the thickest shelf for the upper, pressing one.  If all your shelves are the same size, put a second on top for adequate weight, or add heavy bricks or a steel weight to the top shelf.  (Note: if you use bricks for weights, they need to be dried first.  A two-hour to three-hour soak at 95C should be sufficient.)

Placing

Place the sandbox centrally on the shelf.  If you are doing more than one, ensure there is plenty of space between the pieces and from the edge, so they don’t contact each other, or drip over the edge of the shelf.  The allowances given for the size of the sandbox are a guide.

Two sandboxes placed on separate shelves

Place spacers of the desired thickness around the four corners of the shelf to restrict the extent of thinning.  This also regulates the evenness of the glass across the whole surface.  Usually, 6mm is a desirable height for the pressing.  Other thicknesses can be chosen for different purposes.  The spacers can be steel washers, although they will spall in the cooling stages of the firing.  If you have pieces of ceramic of the desired height, they can be used.  Fibre paper stacked up to the appropriate height are surprisingly robust spacers.  They also provide a cleaner set of spacers than steel.

A corner of the shelf with the 6mm fibre spacer

Place the upper shelf gently down onto the glass piece. The glass at this stage is taking the whole of the weight of the pressing shelf.  The shelf must be placed both gently and evenly down onto the glass to avoid breakage.

Check that everything is in place. This may require additional, directional light such as from your mobile phone or a torch.  It is now ready to fire.

The Firing

This assembly of materials has a lot of mass.  It is 2 to 3 times the normal mass for a standard firing.  

Pressing shelf placed on top of the glass sandbox

This promotes variations in practice:

·        Even with this additional mass, you can fire quickly.  This is because the glass is in small pieces and that the mass of the shelves gains heat slowly. 

·        The greater mass does require longer soaks than a normal fuse firing. 

·        The upper temperature for a full fuse is required to get the glass to a sufficiently low viscosity to allow the glass to move.

·        The long soak at the top temperature does not promote devitrification as in normal fusing.  My speculation is that the glass is not exposed to the air, so the devitrification cannot form. 

·        A further difference in a pressing firing is that the annealing can be at the rate for the final thickness of the glass.  The mass of the shelf and weights above the glass means the glass is cooling evenly from both sides, unlike normal fusing.  The glass may be cooling more slowly than programmed, but the programmed rates limit any possibility of too rapid a cooling.

A schedule for a 12mm thick Bullseye piece with a 19mm upper shelf might look like this:

300°C/hr to    670C       for   180 minutes

300°C/hr to    816C       for   180 minutes

AFAP       to    482C       for   180 minutes

55°C/hr   to    427C       for   0 minutes

110°C/hr to    370C       for   0 minutes

200°C/hr to    50c         for   0 minutes

Off

A piece of 19mm should be slower:

150°C/hr to    670  for   240 minutes

150°C/hr to    816  for   240 minutes

AFAP       482  for   240 minutes

45°C/hr   to    427  for   0 minutes

90°C/hr   to    370 for   0 minutes

180°C/hr to    50    for   0 minutes

Off  

The schedule for glasses other than Bullseye only needs to have the top and annealing temperatures altered to the ones appropriate to the glass.

Results

The pressed glass will have the texture of the shelves on both sides.  Normally, no kiln wash will be stuck to the glass.  If there is kiln wash to be removed, you can do this by abrasive means – sandblasting, diamond pads, wet and dry sandpapers or Dremel style tools.  It is important to keep the glass damp during this process.

If the surface of the glass is without sticking kiln wash or other marks, you can use it with the matte surface.  You can also fire polish the piece, once you have thoroughly cleaned it.

Alternatives

Tape box together

After super gluing the bottom and side strips, you can bind the box together with clear Sellotape.  Pull off at least three strips of tape and set them where you can reach them easily.  Place the upper sheet on the prepared base. Move the box to the edge of the work surface so a little of the box hangs over.  The first stage is to place a strip of tape at right angles to the side to bind the top to the bottom.  Do this for each of the three sides.  When the top is securely attached to the base and sides tape along the length of each of the three sides. 

This shows on the lower left a loosened piece of sellotape on the edge of the sandbox.

This process avoids any difficulty in attaching the top.   Attempting to use only Sellotape to bind the box together is very difficult and requires at least three hands.

Spacers for the frit

Spacers do not always need to be strips on edge.  The spacers can be one or two wider strips placed on their sides to provide the needed height.  They can be coloured, forming a border; but remember the border will become curved. The strips will need to be glued to the back.  The top can be attached with super glue, or taped to the sides and back.

Pressing without a box

It is possible to use the pressing technique without a box or frit.  You can arrange clear and coloured cullet on the shelf.  The arrangement needs to be such that there are no gaps between the pieces.  This means that the glass will probably be 3 to 4 layers thick.  Be careful to avoid creating thick layers of dark colour by interfiling clear. Place the spacers at the corners of the shelf in the thickness desired and fire.  The slower rate of firing (as for 19mm) should be used.

This sandbox process is a combination of arranging frits and pressing.

Try It

What would happen if I tried ... in kilnforming?

A lot of discussion on the internet seems to be a search for certainty in both stained glass and kilnforming. It seems to involve a search for rules in assembling work, or the right way to do a process. The fact is, there always are multiple ways to do a kilnforming operation and still get the same result. Never/Always statements are inevitably wrong in any circumstance that deviates from the occasion(s) that led to the statement.

Rules properly relate to the physical and chemical properties of glass because they cannot be avoided. But manipulating those properties enables surprising results to be achieved. For example, keeping the viscosity stable while extending the time can develop the glass in the same profile as decreasing the viscosity over a short time.

The possibility of an experimental approach is shown by the general acceptance that running tests of fusing profiles is a first step in learning about how the kiln temperatures provide desired results. This kind of experimentation can be extended by using slower and faster ramp rates on the same kind of test tiles to show the effect of rates on results at the same target temperature and time. Tests can be run to show the difference that varying the time at the top temperature has on the outcome. These kind of tests give information for a multiplicity of results from the same layup.

Often an idea occurs and the question “what would happen if …” follows. Try it out. Be adventurous and experiment with ideas. It is rare that you will harm the kiln.

Instead of relying only on conventional wisdom, experiment. See what works and what gives problems. Check the boundaries of the rules you receive. Your skills will develop quickly by trying out your ideas before immediately turning to the internet for opinions on the probability of success.

Be adventurous!

Loctite Glass Glue

Loctite Glass Glue

This is best for small craft items, adhering glass to glass and glass to metal. It is a cyanoacrylate glue.

• Suitable for light items.

• Once cured it is waterproof, transparent, and rigid.

• It is said to be dishwater proof, but will not withstand boiling water.

• No clamping of the pieces is necessary, but the joined object must be left undisturbed for 24 hours. Maximum strength, water and heat resistance is achieved after one week.

• Strong in tension, weaker with sheer stress, i.e., hard to pull apart, easier to slide apart.

• BUT

• Not suitable for glass to plastics, silicone, rubber, or bone china.

• Not suitable for bonding glass to porous materials.

• Not suitable for weight bearing loads or continuous outdoor use.

From the Technical Data Sheet:

• Do not use cured objects in ovens or microwaves

• Not suitable for assemblies which will be exposed to temperatures over 180°F (82°C)

• Do not use on dusty, dirty, or wet surfaces

• Not suitable as a general purpose adhesive

• For indoor use only

• Not suitable for gap-filling applications (parts must be close fitting with no gaps or voids, so roughing of surfaces is unsuitable for good bonds.)

Characteristics

• Application Temperature: Apply above 50°F (10°C)

• Fixture Time: 15-60 seconds, depending on amount used

• Handling Time: Leave undisturbed for at least 10 minutes.

• Cure Time: 24 hours, but times may be needed, dependent on temperature, humidity, porosity of surface and amount of adhesive used.

• Flash Point: 176°F (80°C) to 200°F (93°C)

• Shelf Life: From date of manufacture (unopened): 18 months

• Storage: Store in the unopened container below 90 °F (< 32.2 °C). Not damaged by freezing in the unopened container.

• Once opened, it best stored tightly sealed in a dry cool place, but not cold or frozen.

• Humidity and high temperatures can decrease shelf life.

Colour: Clear to light yellow

Service Temperature: Up to 180°F (82°C)

Cured form: Non-flammable, brittle solid

Moisture Resistant: Yes

Tools Typically Required: Tissue Paper, and a razor blade or scalpel.

Safety

Safety Precautions:

• Use in a well-ventilated area.

• Wear a dust mask and safety glasses.

• Nitrile gloves recommended. Do not use PVC, nylon, or cotton gloves.

• Care in use is needed as it is like water in consistency, and bonds skin instantly.

• Skin contact may cause burns.

• Wash hands after use.

Handling:

• Prevent contact with eyes, skin and clothing.

• Do not breathe vapour and mist.

• Wash thoroughly after handling.

• Avoid contact with fabric or paper goods. Contact with these materials may cause rapid polymerisation which can generate smoke and strong irritating vapours, and cause thermal burns.

• Use an approved hazardous waste facility for disposal.

First Aid:

• In case of eye contact, flush with water for 15 minutes; call a physician.

• For skin contact, flush with water.

• For ingestion, do not induce vomiting; call a physician.

• If spilled on clothing, flush with large quantities of water.

• For bonded skin apply cooking oil to bonded area of skin. Gently message bonded fingers back and forth to “peel” apart. Use rolling, peeling motion – do not pull. Once apart use high moisture hand lotions to aid in removal of excess glue from skin.

Preparation:

Tools Typically Required: Tissue Paper, razor blade or scalpel.

• Surfaces must be clean, dry and free from oil, wax and paint.

• Wash all glassware with water and dry. Avoid using detergents to clean glassware as detergent residues can affect the quality of the bond.

• Protect work area. Pre-fit parts to be joined. All parts to be bonded must be close fitting with no gaps or voids.

• Screw the cap and nozzle down all the way clockwise to tube shoulder, puncturing the tube. Remove the cap counter clockwise from the nozzle. To prevent accidental spillage, do not squeeze tube while removing cap to open.

Application:

• Apply Loctite Glass Glue sparingly to one bonding surface only.

• Immediately press surfaces together, ensuring a perfect line up with no gaps.

• Hold in place for 15 to 60 seconds until bond sets.

• Do not reposition parts.

• Leave parts undisturbed for at least 10 minutes or for best results leave 24 hours for a full bond strength to develop.

• Wait 1 week before washing the item in a dishwasher.

• Clean nozzle tip with tissue and replace the cap, screwing on clockwise.

Clean-up:

After cleaning, wet any tissue used for wiping off glue with water and dispose of.

Use an approved hazardous waste facility for disposal.

Clean up uncured residue with acetone.

Cured adhesive may be cut away with caution using a sharp blade.

After gluing is finished, wash hands.

Based on SDS, TDS, product descriptions and reviews

How to make vermiculite moulds?

 Vermiculite moulds have versatile applications as custom moulds both for draping and slumping. With care, they have a long life. They are relatively light for their size and strength. As the vermiculite is an insulator it does not store heat, making it useful for large moulds without the requirement for long cooling times.  The relatively low cost makes large moulds more affordable.

It is a wet moulding process, so the cast needs to be waterproof. In this example a body cast is going to be used as the form for the mould. The first thing to do is to calculate the volume of material required. For an irregular form such as a torso, I measure the surface distance of the largest part of the mould - the hips in a torso - and the length. This gives an approximation of the surface area of the mould. Then the depth of the mould material needs to be added. All these measurement should be taken in centimetres. The longer the mould and the fewer curved forms, the greater the depth of the mould material needs to be.

In the case of this tall cast, the height of the cast is 96cm and the distance from one side around to the other side of the hips is 45cm. This is an area of 4320 square cms. The depth of the cast was decided to be 5 cm. So the volume of material required is 21,600 cc, or 21.6 litres. This shows the advantage of using the metric system as fewer calculations are required to obtain volumes than when using the imperial measuring system.

Another bit of versatility of vermiculite moulds is that you can make them hard or soft. The softer ones are easier to carve and shape subsequent to the casting of the mould, but are easier to damage.

A soft mould will use a ratio of 6 parts vermiculite by volume to 1 part of cement fondu. A hard mould will use a ratio of 3 parts vermiculite to 1 part of cement fondu. You can of course vary your proportions as you wish within these limits. Any more cement fondu than 1:3 and you are both increasing the cost and the heat retention. Any less than 1:6 and there is the danger that the mould will crack easily and fall apart.

Measure out the vermiculite and cement fondu and mix them when dry. The first picture shows the beginning of the mixing.  During the dry mixing a dust mask should be worn, as the rising dust is irritating to nasle passages. 

The picture below shows the two ingredients fully mixed

You then mix in the water to give a stiff mix. For a hard mould start with water of about 3/8 the volume of the dry mix. For a soft mould start with about half the volume of water to the volume of the dry mix. You will need to test the stiffness of the mix before filling the cast. It should be easy to make a ball in two hands that will stick together, but when pulled at, breaks apart cleanly.

If you can squeeze water out of the ball, then the mixture is too wet. In this case you need to make up another portion of the dry mix and then incorporate that with the already wet mix.

The cast needs a separator to keep the vermiculite and cement fondu mix from grabbing the mould. Vaseline works well, as the mould mix is a wet process. The separator should be spread liberally and evenly over the mould taking area.

Pack the cast by pressing the wet mix firmly down into/onto the cast to get good compaction and conformation to the surface without surface pitting. Build the mould material up in thin layers to ensure even coverage.  You can incorporate some re-enforcement within the mould if you like, such as chicken wire.

The packed cast should be covered to keep the whole damp. Leave this for at least one day. The mould and its cast should be air dried for at least one more day before taking the cast from the mould.

By this time, if you are careful, the mould and cast can stand while air drying.

Once the mould has air dried for a while, you can take the cast off the mould. It is at this point that any gross defects to the mould can be repaired. Any protrusions can be taken away with a coarse or open rasp or other wood working tool. If there are areas to be filled, a small amount of the vermiculite and cement fondu mix can be added. The mould needs to be wetted and the repaired area covered for a day, and then air dried for another day.

The mould should be kiln cured to set the cement fondu. This needs to be done at at least 540C/1004F.

To avoid cracking the mould, you need to soak the mould at 90C/194F for a time - dependant on the size of the mould - before taking it up to the top temperature. I normally use 720C/1328F, which is beyond the slumping temperatures for any glass that I may be using in the future. Once the top temperature is achieved the kiln can be turned off and allowed to cool at its own rate.  This curing process is smelly and so should be done in a well ventilated area, or overnight so the smells do not become a nuisance.

This imshr shows the cured mould, which is much lighter with the free and chemical water evaporated.

After firing to cure, the detailed work can be done on the dry mould. This requires finer grained tools than were required for the un-cured mould. When the surface is as desired, the mould can be kiln washed. The mix that I use is twice as thick as used for shelves to reduce the amount of water that needs to be evaporated from the mould. The drying should occur at 90C/194F until no moisture comes from the kiln ports.

These images show the kiln washed mould ready for placing the glass

Custom Moulds

What can I make custom kilnforming moulds from?

There are a wide variety of moulds available to purchase, but sometimes they are too expensive, or a special, distinctive form is wanted and needed.  There a number of ways to make a custom mould with a variety of materials.

Fibre Paper (including SilkeMat)

These are the most flexible in terms of shape and ease of use.  Most fibre papers range from 1mm to 6mm. Fibre blankets range from 12m to 50mm. They can be shaped directly or pressed into or onto existing forms.   Often, the fibre papers become more flexible after firing, although more delicate.  When combined with a hardener, they can provide nearly permanent moulds if treated with care.

Fibre board

Fibre boards are semi-rigid pressed refractory fibres which will maintain their form but become fragile after the binders are burned out.  Common thicknesses for fibre board are 15 to 25mm (0.625 to 1.0”).  They can be used as simple moulds, bottomless bowls, single drop out rings, and multiple drop outs without needing any separators.  The advantage of these is that when the base of the formed glass becomes larger than the hole, the board can be broken away from the glass without damaging the glass.  The mould is destroyed, of course.  However, when rigidised the fibre board becomes a strong, permanent mould.  It will need to be coated with a separator to prevent glass sticking.

It is difficult to obtain much detail with fibre board as the mould, and any glass resting on the board will take up the texture.  Once rigidised, it is possible to sand it to a smoother surface, but the surface will always have a texture.

Vermiculite

Vermiculite board is much firmer than fibre board, and is available in 25mm and 50mm (1” and 2”) thick boards.  It must always be kiln washed before any use because of the mica content.  The vermiculite board can be cut and shaped with wood working tools, but makes the tools unsuitable for woodwork afterwards, so the tools should be kept separately.  It can be worked and carved to obtain a lot of detail.  It is reasonably robust even though it does not require rigidisation.  It can also be machined with CNC tools.

Investments

Investment moulds are usually a combination of plaster of paris, silica, and strengthening materials such as chopped fibreglass or grog.  Generally they are one-use moulds, although there are a few formulations that will last a number of firings.  The instructions for mixing and moulding are widely available.

Found moulds

The most common found moulds are ceramic or stainless steel kitchen or table ware, but they can be of any material that will withstand the temperatures required for the glass. Glazed ware will normally need roughening or removing the glaze before adding a separator.  However boron nitride (Zyp is a trade name for it) often can be applied directly over the glaze or metal.  Casts can be taken from other less robust materials to form investment or other mould materials around them.  Room temperature vulcanising (RTV) rubber is a good material to obtain a lot of detail.  If less detail is required, fibre papers can be formed around the object with a separator of some sort to prevent sticking, and rigidised.

Customising existing moulds

Moulds can be modified in a number of temporary ways.   Adding a fibre paper design to the sides and edges can emboss it into the glass.  A mould with a round bottom can have a disc of fibre paper or a flat layer of kiln wash powder placed to give a flat bottom.  Square glass can be put into a round bowl to give a dramatic appearance, and round glass can be put into a square mould for a different look than in a round mould.  The variety of modification is limited only by the imagination.