Wednesday, 30 May 2012

Fibre Blanket Moulds

Fibre blanket moulds are good for free form moulds. The blanket can be cut into shapes or crumpled. It does not have binders as the papers do, so kiln wash is not necessary. Still, I have always sprinkled alumina hydrate powder over the mould. You can then support the high spots with kiln furniture – existing or custom made.

Pre-wetted fibre blanket is available - Moist Pack is one brand name.
Or you can make the mould yourself from fibre blanket and hardener. You need:
  • ceramic fibre blanket. It should be 3 mm or thicker, but 25 mm needs to be compressed when wet. It is possible to use two layers of 3 mm fibre blanket, but they do not stick together well unless thoroughly wetted.
  • colloidal silica - often is called mould hardener or rigidiser. Paint this onto the fibre blanket liberally, both sides if possible.

The rigidiser can be brushed on or sprayed on. Some people soak the blanket in the rigidiser and then squeeze out the excess.

You must protect the master with cling film, Vaseline, or other waterproof separator. Be sure about whether you want a draping or slumping mould, as the inside needs to be smoothest for a slumping mould and the outside smoothest for a draping mould.

Press the wet fibre blanket to the master. Then let it dry for a couple of days to become stiff enough to remove from the master. Let the negative dry for another period when out of the mould.

The drying method for rigidised fibre mould depends a bit on the structure from which you are taking the shape. If the shape is a piece of glass you can heat slowly to about 300C, but you have to be careful not to go much above that temperature to avoid the mould sticking to the glass. When cool you can carefully remove the mould from the glass and fire it to about 720C to cure it.

Other materials should be able to withstand at least 400C if you are drying in the kiln.

Materials that cannot be subjected to heat should be air dried. This will take a long time, possibly a week or more. The master should be coated with petroleum jelly or cling film to ensure the drying of the mould does not also cause it to stick to the master.

When the mould is dry, put into the kiln and fire to around 760C to cure the mould. You can fire fast, and after 10 minutes at 760C, you can just turn the kiln off, as there is no possibility of thermal shocking the mould. The point is to get the glass which has been in suspension to soften and stick together. Upon cooling the mould will be hard, as it is held together by the glass structure within the fibre blanket.

Once rigidised, you can sand the mould to refine the shapes. But you must use dust mask as the dust and fibres are dangerous to your health.  Do it out doors if possible. Otherwise a well ventilated room is necessary. You can sand down the high spots and generally smooth the mould to obtain a finer texture. Usually 100 grit sandpaper does the job quickly and leaves a relatively fine surface texture. If unhardened blanket is exposed during the sanding process, You can add a mixture of the rigidiser and the "dust" from the sanding to any holes, dimples or exposed unhardened fiber in the mould Then re-apply rigidiser to the sanded areas, and cure the mould at 760C again.

If you are rigidising, you need a separator – kiln wash – either powder or in a solution brushed on. A rigidiser does not burn off; it fuses to itself within the mould material and makes it harder. The resulting mould material will also be more brittle and should be handled with some care. I.e., never pick up the mould by the edges or with a piece of glass on top.

When you are satisfied with the shape and texture, you apply the kiln wash and fire.

The rate of heating the kiln and the soak will depend on the complexity of the shape of the mould and the thickness of the glass but there are no concerns about the mould as it is not subject to thermal shock.

With delicate treatment, the mould can be reused many times.
Two examples are shown here:
Lamp shade panel form

This is a "free form" mould made to give the glass sheet the appearance of crumpled paper