Mortars & coatings

Airset mortars

Mortars used for refractory work are different from those  used in domestic and commercial construction.

Those used in kilns for glass and pottery are called ‘airset’ mortars.

They are sold premixed, usually in sealed steel pails of about 15kg capacity, are of a darkish colour with a reasonably fine texture and, when first opened will have a layer of liquid on its surface.  This must be thoroughly mixed back in, preferably using a mechanical stirrer and working the liquid back throughout the pail.

They have a shelf life of up to about 9 months if kept completely sealed in containers which are nearly full so as to exclude most of the air.

To come

Fig 61-01

An effective tool for stirring is a bent steel rod in a portable power drill, as shown in Fig 1

Like most mortars, they thicken up fairly quickly if left undisturbed in the air (the initial set, which is reversible by stirring), so must be stirred every few minutes to keep them at their best working consistency.

To avoid having to frequently stir the whole pail, decant the quantity needed for the current work into margarine or similar containers and reseal the pail. The small containers can be sealed until required and are easily stirred using a paint scraper as a combined stirrer and applicator.

Most IFB are quite absorbent. They will speedily absorb the liquid from the mortar, making it too stiff and dry to conform and bond to the mating surface; a dry joint can result.

Working fast and stirring the mortar frequently can usually overcome this.

If necessary, a small amount of water can be added; but be very sparing, as a little goes a long way.

Alternatively, a small quantity of sodium silicate (waterglass) can be added to put some life back into a stale pail.

Patching mortar

Buying a pail of refractory mortar to make small repairs is wasteful and unnecessary. A mix small enough to do the job in hand can be made up at any time using readily available materials; kaolin and colloidal silica.

• In a small margarine or yogurt container with a sealable lid place a small amount of kaolin. The grade is not important.
• Add full strength colloidal silica, a small quantity at a time, and mix in thoroughly until a smooth paste is produced.

Keep sealed and the mix will last many months. The mortar will dry out by the loss of water, so add more water from time to time to mmaintain it at its original consistency. Once dry it is set permanently.

This mortar dries quite hard, but nowhere as hard as the commercial refractory mortars.

It can be used for applications such an alternative to the refractory mortar shown in Making a fibreboard mould, or for repairing a crack in a hearth or chips falling out of brickwork.

Coatings

Here we refer to materials which can be applied to the hot face of kiln linings.

At glass temperatures we are mainly concerned with locking up surface fibres in CF linings, to prevent them falling onto the glass.

Colloidal silica

The most suitable material for this purpose is colloidal silica, sprayed onto all exposed surfaces.

Most suitable device for spraying is the type of pump-up pressure spray used for pest control in gardens.

The colloidal silica sold as Rigidiser or Hardener by refractory merchants can be diluted about 50% with water and applied as a fine spray slightly heavier than a mist, so that the fibre surface feels lightly wetted.

Colloidal silica cures by the drying out of the water by evaporation from exposed surfaces. Gentle heat can speed up the process.