Colloidal silica

What is it?

Its the most common inorganic binder used with ceramic fibre. It also has wider applications in kilnforming.

Essentially, it is ‘ionised microfine silica particles suspended in water’. The particles are so fine that the small electrical charge on each one allows them to repel one another and remain in suspension in the water.

Only a certain concentration of particles can remain in suspension. When this is exceeded the surplus forms into crystals and falls to the bottom of the container. Once re-crystallised, the material will not go back into suspension.

If the liquid is frozen it will be no longer usable after thawing. Freezing point is about 0°C (32°F).

The most commonly used form is slightly alkaline so can sometimes irritate delicate skin. pH is about 9.5

It is packaged by Thermal Ceramics as ‘Hardening fluid’, and by Unifrax as ‘Rigidiser’ and is available from them or their stockists in containers of 1 litre and upwards.  

It is used in wet CF (ceramic fibre) blanket products such as ‘Moist Pack’ and ‘Wet Felt’ and is the long lasting binder in fibreboard and CF papers.

The water is only the vehicle to keep the particles in suspension and plays no part in bonding fibres together. Its use rating is as high as the fibres with which it is used. However, where shrinkage cannot be tolerated its use should be limited to about 900°C (1650°F). 

Once dried out, colloidal silica is unaffected by water or most solvents. It will not soften if re-wetted so surfaces treated with it can be battwashed without detrimental effect.

How does it work as a binder?

As well as repelling one another, the silica particles are attracted to surfaces with a different electrical charge. Thus, when the liquid is added to a fibrous material such as CF blanket, a first layer of particles will be attracted to the surface of the fibres. This has the effect of roughening up the surface, making it less slippery and more easily entangled one fibre with another. 

Any additional particles will be repelled from the surface layer on the fibres and remain is suspension in the water. The water evaporates from the exposed surface of the blanket, drawing surplus particles with it and depositing it there in the form of silica crystals. The result is a crusty exposed surface and a more fluffy interior.

Where and how to use it?

On fibreboard moulds and similar items which have lost strength after the binder has burnt out, apply by brush direct from container to give it permanent strength.

It can also be applied by spray gun or atomiser spray. A pump spray of the type used for garden spraying is preferable to a trigger spray type.

The spray unit should be washed in clean water immediately after use: otherwise a dry residue will remain and be hard to remove.

Use a soft brush to remove the bulk of the battwash and then apply the colloidal silica. It will bond the battwash to the board. Of course, the battwash will no longer be effective as a separator and fresh coats of battwash will need to be applied before the item can be used.

Allow the first coat of colloidal silica to dry out thoroughly from the inside before applying another coat if considered necessary.

Drying in a kiln can be done if necessary, but no hotter than about 120°C. Air drying gives better results, even if it takes longer.

A crust can be given to a peeled layer or torn piece of CF blanket.

CF blanket is often used to impart form or texture to kilnformed pieces. Colloidal silica can be used to give it strength and rigidity, and to maintain a shape.

Place the blanket piece on a sheet of polythene film and apply the colloidal silica liquid. The polythene will prevent the blanket sticking to bench top or other support and can be peeled away when dry.

Be sure and battwash after the coating is dried.

A spray coat can be applied to the fibre hotface of a glass kiln to lock up any loose fibres and prevent them falling onto the glass.Easiest and most economical method is with a low pressure spray such as a horticultural or pump spray. Can be diluted with clean water to a specific gravity of below 1100.Sensibly, in this case a mask would be worn; as would be the case whenever doing either low or high pressure spraying of any kind.

It can also be used as a binder with kaolin to make a soft mortar for filling cracks in  brickwork. Mix an easily spreadable paste and sand smooth after drying.

Do...

• If crystals appear in bottom of container, add more water to replace that lost and to lower the solids concentration. Make sure container is well sealed.
• Wash out brushes thoroughly after use. Any residue will set hard.
• Use gloves, or wash hands after use to minimise skin irritation.

Don’t

• Allow to freeze.

Health aspects

Because of the hysteria engendered by the word “silica”, generally due to misinformation, some people won't use colloidal silica; even in situations where it cannot do harm.

Whilst it is true that free silica particles floating in the air could be inhaled and would then constitute a health hazard; the silica particles in colloidal silica are not released into the atmosphere in this way. IF sprayed, they would be part of water droplets which would rapidly fall to the ground.

It can be reasonably said that; “applying colloidal silica solution by brush, or by low pressure spray whilst wearing a mask, poses less risk than walking in the open air on a dusty day”.

In the latter case the airborne dust can consist substantially of dry silica particles. 

Storage

Colloidal silica dries by the evaporation of the water vehicle, so store in a sealable container.

Screw top glass containers offer advantages and problems. When first used they can usually be sealed without difficulty, but after being opened a few times there will be a residue on the lid and mating surface on the jar which will prevent a proper seal. However, being transparent, the liquid level will be visible and can be marked on the outside; so that water loss can be monitored. Simply add clean tap water from time to make up the loss. It will keep in storage for years.

Occasionally a fungus will grow on the surface of the liquid. This can be removed by straining through a fine cloth or pantyhose, or filter media of similar porosity. Minor particles pose no great problem as they are organic and will burn out at a low temperature when the applied coating is first fired.