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.