Ceramic fibre blanket paper & board

Alumino-silicate fibre products

Products made from blown or spun alumino-silicate fibres play important roles in kilnforming. 

• They can be formed into blanket for kiln insulation or as a packing material for impromptu moulds.
• Used in place of wood fibres in paper making, they produce a range of high temperature papers of varying thickness and texture.
• When used with organic and inorganic binders and refractory fillers they can be made into boards blocks or many unique shapes.

Before looking at the product forms used by the kilnformer we should first have a brief look at the fibres themselves.

Ceramic fibres

Belonging to the group of materials known as Synthetic Mineral Fibres (SMF), ceramic fibres are made by melting the mixed raw materials (alumina & silica sand, or kaolin) at a high temperature and then propelling small droplets at high velocity so that some will become like a ‘comet with a tail’. The tail becomes the fibre. They have a smooth slippery surface and are brittle, just like most glass.

Other particles will tumble instead of flying straight and become small glass beads called shot.

Some fibres are so fine that they bypass the normal processes and are caught in the fibre plant dust collector system. For more, see 12. Ceramic fibre linings.

CF Blanket

The bulk of the fibres will fall onto a conveyor belt where they form a matt which is needled for strength and supplied in rolls of differing thickness and density.

Many kilnformers keep some 13 or 25mm (1/2” or 1”) thick blanket handy; sold by the metre by some potters suppliers or in rolls by refractory suppliers.

CF papers and boards

Unlike natural fibres, CF’s have a smooth shiny surface so they don’t easily entangle one with another. Binders or glues must be used to hold them together.

These binders fall into two groups;

• Organic binders, and
• Inorganic binders.

Organic binders

Are ‘glues’ which break down and lose their strength when first fired to a low temperature.

Some organic binders are made from vegetable matter such as potato starch, whilst others can be made from crude oil, gas or coal.

Whatever their origin, they are all combinations of carbon and hydrogen and will lose their strength on the first firing at between 150ºC and 250ºC (300°F and 480°F): when the hydrogen combines with oxygen in the air to make water vapour. The carbon remains and forms a grey or brown deposit.

With further heating the carbon combines with oxygen to form carbon dioxide gas and the staining will disappear: generally between 400ºC and 500ºC (750°F and 930°F).

This staining can sometimes be seen in materials containing organic matter when its position prevents it reaching a sufficiently high temperature, (or receiving sufficient oxygen) to burn it out. It is sometimes seen on the bottom surface of fibreboard moulds. 

Some binders give off smoke and a pungent odour (it stinks) early in the first firing.

The emission of fumes should generally be complete by about 400ºC but a safe practice is to leave a vent open to about 500ºC.

Inorganic binders

These binders don’t break down at kilnforming temperatures.

The most widely used is colloidal silica,composed of microfine particles of silica held in suspension in water.

When applied to the board or paper, a single layer of silica particles will bond to the surface of individual fibres. Any surplus particles will migrate with the water to a surface from which the water can evaporate.The remaining silica particles form a heavier deposit on the outside surface; creating a crusty surface with a softer inside.

Colloidal silica has many uses in kilnforming. See 39. Colloidal silica for more.

Other inorganic binders include colloidal alumina and sodium silicate.

CF papers

Made on machines similar to those used for making newsprint, CF papers can vary widely in composition texture and fired strength: as well as in price.

The mixture of ceramic fibres and organic binders in massive volumes of water are flooded onto a stainless steel conveyor belt, the water drains through and the glue coated fibres remain. The damp web of fibres is passed through rollers to give it uniform thickness and driers to set the binders.

The paper can be made in various grades.

The cheapest grade will be made using every part of the fibre, including the shot: so it can be ‘gritty’ with small lumps, especially the thinnest sheet. It may have a ‘smooth’ side and a ‘bumpy’ side.

More expensive grades have had the shot removed, so have a smoother and more even texture on both sides.

The amount of organic binder can vary considerably, as can the degree of compression it receives during manufacture. All this affects its behaviour in the kiln and its ability to withstand repeat firings. The only way to tell how re-usable a particular paper will be is to do a test firing.

CF papers are made in nominal 1/32 inch, 1/16 inch and 1/8 inch (nominal 0.8mm, 1.6mm, 3.2mm) thicknesses. As the thickness tolerance can be wide, some outlets refer to them simply as ‘thin’, ‘medium’ or ‘thick’.

Most thick paper will tolerate some small number of repeat firings if left undisturbed: whilst the best quality of the thickest grade may well survive indefinitely if not disturbed.

The residual strength after firing decreases with reduction in thickness, so that the thinnest is rarely reusable more than once or twice.

Because of the small quantity involved it is usually not necessary to burn out the binder from CF papers before using them under glass. Some papers lose all their strength or fall to pieces after the first firing so it would be pointless to pre-fire them.

At the top of the range, price wise, is Bullseye ‘Thinfire’ paper. It is extremely thin, contains no shot and only the shortest and thinnest of fibres, has minimal organic binder and disintegrates after one firing.

Ceramic fibre paper may be used for a number of reasons, and the use can determine how long it lasts: or how many firings it survives.

• If it is chosen to impart a particular texture to the glass then the number of firings it survives will depend greatly on how the texture itself survives.
• If it has to be removed after a firing it is very difficult to maintain the texture. It may be best to consider it as a one or two firing material.  Rolling it round a cylinder of reasonable diameter straight off the hearth can help extend its life.
• If it is being used to smooth out unevenness in a brick hearth or to bridge gaps in a multi slab hearth, then a good quality thick paper layer can withstand innumerable firings. This is especially so if a texturing powder is sprinkled on the paper so that the glass doesn’t actually contact the paper and disturb the surface.

CF board, fibreboard

Made in a vacuum forming process, so sometimes called ‘vacuum formed board’ or ‘vac board’, it consists of ceramic fibres held together with organic and inorganic binders.

Made from 5mm to 75mm thick but most commonly used by kilnformers is 10mm or 12mm for moulds and 5mm as a heavy underlay.

It is no longer made in Australia and is imported in sheets as large as 1200 x 1000, that's mm of course. Because it cracks fairly easily it is best to buy it in smaller sheets which are easier to pack and transport.

Board can vary widely both in surface finish and in fired strength. Before firing, the strength comes from both the organic and inorganic binders, but after the first firing the organic binder is gone. It can be high on organic binder and low on inorganic. The green strength of a board gives no indication of how strong it will be after firing. Only firing a sample can show that.

Usage of fibreboard is not high and the kilnformer will need be content with what is available in their area, irrespective of quality. Fortunately added fired strength can be given by applying additional coats of colloidal silica. For more on this see 13. Making a fibreboard mould.

There are two ways of making the board. One uses a flat form the size of the sheet, onto which the material is sucked and from which it is expelled onto a flat drying surface; one sheet at a time. The other uses a continuous process called Roto-forming, where the material is sucked onto a drum and fed continuously onto a flat conveyor belt for drying and cutting to length.

When fibreboard was made in Australia it was by the former ‘batch’ method, but the imported product is more likely to be made by Roto-former.

When using a flat piece of fibreboard, say 10 or 12mm thick, as an underlay, it has been noticed that when the piece is large there is a tendency for the corners to lift; so that a fired piece of glass won’t be perfectly flat, but slightly dished. There is a suspicion that the lifting is greater with Roto-formed board than with the previously available material.

Another kiln builder who uses fibreboard as a hotface layer at glass temperatures has recently commented on the dishing which is occurring. Could it be that the flattened-out Roto-formed board is trying to curl up again.

To summarise;

• When new, the strength of the board is determined by the concentration of both types of binder: the proportions of which can vary widely.
• When fired, the organic binder has gone and only the colloidal silica gives it strength.
• Starch is much cheaper than inorganic binders, so some makers may scrimp on the colloidal silica and their fired product can be weak and fluffy.
• If making complex fibreboard moulds it is best to pre-fire them.
• If using board for moulds, fire an offcut first to find out how strong the fired piece will be.  If it’s weak, or the surface isn’t firm, give the moulds one or more coats of colloidal silica to harden them up. Dry between coats. When fully dry they can be battwashed.