Moulds

Any form or shape capable of withstanding glass firing temperatures which can be used to give shape or form to glass. This encompasses a vast range, from piled up powder over which glass is draped to the most complicated glass casting mould.

Moulds fall into two categories:

1. Those for slumping or bending relatively thin sheet glass, and

2. Those for casting where the mass is high and where the pressure on the mould can be great. These will be dealt with in  63. Glass casting. 
   

 Moulds for slumping or bending

 These can be found objects or items especially made for the task.

They can include;

• Shapes cut from refractory board such as CF board or vermiculite board such as Ceramaguard.

• Shapes cut from Silimanite slab or batt.
• Hand formed or slip cast pottery shapes.
• Items such as table crockery porcelain or china bowls.
• Spun or folded metal shapes.
• Plaster or silica/plaster shapes.
• Ceramic fibre blanket.
• CF castables.
• Found objects such as sea shells, leaves and twigs.

Refractory board

The most widely used boards are CF board and the vermiculite board called ‘Ceramaguard’. They both make re-usable moulds and can be carved with designs on the flats. Because of their texture the detail won’t be very fine. They both need battwashing.

See 13. Making a fibreboard mould, for more.

Calcium silicate board has a fine texture and can accept finer detail when carved. It does NOT need to be battwashed.

Silimanite shapes

These can be sawn or water-jet cut from kiln shelves or batts.

Water jet cutting is a process which uses fine abrasive particles carried in an immensely high pressure jet of water to abrade away the material on the cut line. There’s a setting-up charge for each shape to be cut, so the process is best used where a number of identical items are needed.

Batts can be sawn using a diamond saw or ‘stone’ grade abrasive discs in hand grinders or masonry ‘drop saws’. The material can be drilled with an ordinary masonry drill, so complex shapes can be made by drilling a series of linked holes clear of the cut line and breaking out the waste.Trimming to the line can be done using stone grade abrasive discs in ‘angle grinders’ or abrasive ‘burrs’ in power, air or battery drills.

Silimanite, like most refractory materials, is brittle and chips easily, so care should be taken when handling.The dust contains silica, so a mask should be worn whenever a dust is created.

Wet sawing or drilling is advised; not only to suppress dust, but also to avoid the risk of cracking the piece through localised over-heating.Make sure they are battwashed. 

Pottery shapes

These can be factory made and sold through art glass outlets, or custom made by hobby or professional potters.

Slip cast or wheel thrown clay shapes can be used to produce many identical pieces of a design. The moulds require firing to bisque temperature, 950°C, (1740°F) before use. There is considerable shrinkage of the mould during it’s initial firing, (can be up to 15%) so due allowance must be made.

Although there are more specialised clays available, shapes made from the normal slip casting clay mix used by potters and “ceramic” hobbyists works well if properly fired. Moulds pre-fired at about 2.5ºC per minute in a glass kiln have been used many times without failure.

NOTE. Firing pottery shapes is best done by potters as the firing process is sometimes not quite as simple as stated above. Also, depending on the clay body chosen, there can be lots of smell and fumes.

Fig 1

If the mould is to be used to make plates or platters, make sure that the top inner surface is raised toward the centre, so that the underside of the glass will be raised in the centre and the piece will sit flat.

Few potters understand this, so make sure they are instructed thoroughly in what is required.

The top of the mould will be the bottom of the glass piece, so shape it accordingly.

The moulds can have designs carved in or embossed on them whilst in the green state, (before it has been fired) but beware having a design so deep, or at the wrong angle, that the glass piece will be trapped in the mould. Make sure they are battwashed.

Fig 2

Alternatively, a flat ring mould can be used with a disc of CF paper placed in the centre to give a raised bottom.

Crockery items

Table crockery porcelain or china bowls can often be used as moulds but care should be taken to ensure that the finished glass piece has a slightly domed bottom so that it will sit properly on a flat surface.

Flat plates make excellent moulds, as they can be found at Op-shops in a wide range of sizes, often with textured or fluted edges. However, they usually slope in toward the centre, so fix a disc of ceramic fibre paper in the centre to ensure that the bottom centre of the glass plate is raised so that it sits flat.
Do the same if using a crockery bowl as a mould.  

Make sure they are battwashed.

Metal shapes

Wire frames, wire mesh shapes or even a single strand of wire or steel rod can be used as a support over which to drape sheet glass. Long life moulds can be spun, pressed or shaped from Stainless Steel.
Cast iron pieces can be used but aluminium isn’t suitable. Mild steel can be used, but mill scale can be a problem and it rusts badly after use.

Don’t forget the ‘WOK’. At the risk of earning the ire of the cook, one can apply a hammer to the bottom and give it a slight dome, so that the fired glass bowl will sit flat. It won’t be much good for stir fry after that treatment. Don’t try it with Teflon coated, aluminium WOKS or items with wooden handles.

Fig 3

Metal moulds must be battwashed. All except stainless steel will rust after being in the kiln, but can be cleaned up for re-use; rub up with a wire brush or coarse steel wool and re-coat with battwash. Rust will start to form on the cleaned surface immediately the battwash is applied, so the clean-up should be done shortly prior to use. Fig 3 shows two old mild steel woks which had been used and put aside for some considerable time. View on left shows the surface after the loose battwash and millscale had been scraped off whilst that on the right has been cleaned and battwashed ready for a firing.

Don't confuse bright plated mild steel sheet kitchen items for stainless steel. The quality of some plating and polishing can often make it difficult to spot the difference.

Plaster moulds

Plaster moulds can be used but it’s important that they be fully dried before use. To get the finest finish use pottery plaster, obtainable from major plaster suppliers.  The makers of moulds for slip cast pottery items are expert in making this type of product.

Bulking agents or fillers such as sand or kaolin are often added to reduce cost, or vermiculite to reduce mass.

 Handyman packs of plaster based patching products should be avoided as they will often contain organic based additives and plasticiser which could burn out and may cause the mould or form to break up.

CF blanket

Pieces of CF blanket can be cut to create regular shapes, or torn to make irregular shapes such as finger grips on flat platters.

Fig 4

The handgrip on the platter was formed by tearing a piece of 25mm thick CF blanket. When setting up, the entire weight of the glass was supported on only the blanket and the opposite edge and this compressed the fibre piece so that it appeared too thin.. However, when the glass softened the bulk of the weight was removed from the blanket and it sprang back up, so that the height of the lip approached the original height of the unloaded blanket. 

There are many other situations where CF blanket can be used to create interesting and unusual shapes. Don't forget how easily it is compressed and the way in which the weight of glass can shift as it softens.

Hebel Block

This is a relatively low mass commercial building material. It is easily worked with woodworking tools, can be sawn drilled or sanded with common abrasive materials. Cavities can be made by chisel drill or with abrasive points in a power drill. It has a medium texture which can be filled with battwash. 

Found items

These can be any of a wide range of organic or inorganic items.

Items such as sea shells can be used directly, by placing them onto the hearth and slumping the glass over them. Although they are calcium carbonate which doesn't need battwashing, it is best to battwash them as they could have other material on their surface which may stick to the glass.

Alternatively, they can be arranged in a design and have a plaster slab cast over them; and this can become the form into which a sheet of glass can be slumped. This same technique can be used to transfer the shape of organic items such as leaves or twigs to glass.

The range is enormous.

CF castables

 Complex shapes can be produced in moulds made from ceramic fibre based casting compounds such as ‘Unifrax Moldable’ or 'Thermal Ceramics Superfil', obtainable from refractory suppliers. 

Supports for drop through's

Lipped round bottomed bowls can be made by placing a silimanite ring of suitable size on a wire frame or on posts.

For long life the frame can be made from stainless steel rod, but mild steel is OK if one can put up with cleaning up the heat damage after each firing. Be sure and clean off any scale on metal items before use (even if they won't be contacted by the glass) as they can flake off during the firing; and who knows where they will land.

A pattern can be imparted to the flat rim of the bowl by forming the pattern onto or into the support ring.

If the drop through is allowed to touch the shelf it can produce an enlarged bottom, and this may be too large to pass through the hole in the support ring.

Instead of using a silimanite ring, discs of fibreboard can be used and these can be broken away after the firing.

Other methods of providing easily removable support, as well as creating unusual top lips, include using screwed-up chicken wire overlaid with ceramic fibre blanket or paper.

Undercuts, glass hang-ups

It’s important that there be no undercuts or reverse tapers on one piece moulds, as these can prevent the glass piece from being removed from the mould without damage.

Allowance must be made for the shrinkage of the glass as it cools. Differences in the rate of shrinkage of the glass and the mould can sometimes cause undercuts to break off, glass pieces to jam in the mould, or mould or glass to be broken by the stress.

Air escape holes

In deep moulds, always make sure that there is some way for air to escape from underneath the glass as it slumps.

Small holes can be drilled to allow the expanding air trapped between the mould and the glass to escape and to avoid the blowing of bubbles in the glass.  Put these holes in the area of the mould which will be the last to be contacted by the slumping glass. Usually this is in the side and just above the bottom. Use a small masonry bit or diamond tipped drill.

Powder beds for small kilns

Fig 5

A kiln shelf can be adapted as shown above to allow a powder bed to be made whenever required. Strips of silimanite shelf about 20mm (3/4”) wide have been cemented to all four edges of the shelf, allowing the recess created to be filled with the bedding powder of choice. The powder can be returned to the bulk container after use.

The shelf and strips should be battwashed before use.

Alternatively, strips of fibreboard can be used, but these become soft after the binder burns out and are more prone to damage. Giving them a heavy coat of colloidal silica can toughen them up somewhat.     

Cements for mould making.

There are a number of refractory mixes which can be used.

Commercially available refractory mortars such as Tufset Mortar are sold only in pails of about 15kg and, because of their relatively short shelf life are unlikely to be of interest to kilnformers.

Another disadvantage is that they become extremely hard when set after a couple of days and are difficult to cut.

Mixes using colloidal silica or sodium silicate as a binder and kaolin or alumina powder as a filler can be easily mixed in small quantities as required.

The materials are available in small quantities and there is the added advantage that both mixes can be easily sanded or cut with a hacksaw blade; even after firing. See 61X. Adhesives & mortars  for more.