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Glass slumping moulds from clay

The majority of people slumping glass, use one of the kaolin fibre products to produce their moulds. It may be fibre blanket, board or one of the castable products. The Kaolin products are excellent for the rapid heating and cooling phases of the firing cycle, as they are not greatly effected by the silica change, they allow the glass to breath and so release any moisture or impurities the may be present.

The main problem that I find with these products are that:

  1. They have limited life span ie: one or two firings.
  2. They are expensive which increases your production cost.
  3. Handling the products can create dust, which may be inhaled, and the consequences are an unknown.
  • One good thing is that they can be reused if turned into slurry and remoulded.

In the past I have on occasions had people come to me and ask for a bisque plate shape to slump their bullseye onto. These plates being only made from a stoneware body will only take limited amounts of firing, because of this it seems to be assumed that all clay bodies have a limited life span.

Clays in packets that you buy from a ceramic outlet are mixtures of various minerals. They are formulated to suit firing temperature, colour, production method and surface texture. This is what is called a clay body.

Some clay bodies are tight ie: small particles packed very close together. And some are what is called open, ie: larger particles that can't pack close to create a dense body. For our glass moulds we need an open body formulated with the type of materials that can withstand the thermal shock of fast cooling and repeated firing.

From the commercial range of clays the one most suited to our needs is Raku. The various manufacturers have different recipes for their Raku bodies and you might need to try a few before you get one you are happy with.

If you understand a little about ceramic materials you might like to mix your own refractory body for producing your mould. Once you have formulated a satisfactory body, you should be able to get many firings without severe defects in your mould.

Materials that may be formulated into your body included:

  • Fireclay.
  • Talc.
  • Calcined kaolin.
  • Openers such as saw dust, flour or paper.
  • Fine grog.
  • Mullite.
  • Spodumene.
  • Plasticiser such as bentonite, ball clay.

With the commercial raku clays, once you have determined the required thickness it is simply a matter of rolling the clay into slabs, using either slats or a slab roller. Cut the slab to the required size and shape; apply decoration if required, hollow back section and insert breather vents.

Bisque fire.

At this stage sprig mould decoration can be considered. The separating agent can be applied. Surface texture can be applied.

Glass cut.

If you need to make more than one mould for production purposes you will need to make master moulds from plaster of Paris. Into these you can either slip cast or press mould. The main thing to remember is that inner corners are better rounded than square. My personal belief is that press moulding produces a quicker and better glass mould than does slip casting however it is more labour intensive if you are producing your own body.

A good starting point for developing your own body.

  • 50% fine Hallam fireclay.
  • 30% kaolin (10% can be calcined).
  • 20% talc (10% can be calcined).

Further additions to this can include some organic material; this will have the effect of opening the body and allowing the gasses to escape.

This recipe will mix up quite well as a slip with the addition of sodium silicate; it can produce reasonable working clay for press moulds and can be used for layouts for architectural glass.


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