Adhesives & mortars

Adhesives

The adhesives being considered here are those used to temporarily hold glass pieces together whilst they are being assembled into the kiln before a firing.

They will be organic compounds, so that they burn out at a low temperature.

They will be free of inorganic fillers or colourants, so that no residue will remain to marr the fired glass.

At the risk of being repetitious, organic compounds are comprised of hydrogen and carbon. They turn into water vapour and carbon dioxide gas at below 400°C (750°F).

Inorganic materials generally do not burn away at glass firing temperatures and could leave a residue after firing which could be unsightly.

Among the common materials which can be used are:

• flour and water based pastes such as Clag;
• PVA (Poly Vinyl Acetate) woodworkers glue;
• Superglue;
• clear nail polish.

Some dry quicker than others, so choose the one which allows time to manipulate the glass. Even superglue is available with different setting times.

Use only the minimum amount necessary to hold the glass. Most of them dry in from the edges, and when the edges are dry the holding job is done. Not much point in having a blob of wet adhesive in the middle. A small blob on the end of a needle is often enough.

When fusing two pieces of glass with the top piece slightly overhanging the lower, put two tiny blobs near opposite edges; that will hold them in the correct position without risk of twisting or of edges misaligning.

Mortars

The mortars being considered here are those needed to assemble moulds or to make a minor repair to a hearth or kiln brickwork.

Mortars able to withstand kiln temperatures are known as refractory mortars. They are considerably different to those used in domestic and commercial building operations.

They are generally supplied ready mixed in sealed pails of about 15kg and have a shelf life of less than one year. They are too costly and wasteful for consideration by kilnformers.

Fortunately, there is an easy way out. The kaolin used in battwash can be mixed with colloidal silica to make a paste which is an effective mortar for cementing together pieces of fibreboard or vermiculite board for moulds, or to make a small repair to kiln brickwork.

Colloidal silica has lots of uses which are detailed elsewhere in this website.

Home made mortar

• In a small margarine or yoghurt container with a sealable lid place a small amount of kaolin. The grade is not important.
• Add full strength colloidal silica, (usually 10% concentration) 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 to restore it to its original consistency.

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

It can be used for applications such as 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.

As an alternative to colloidal silica in this mortar mix, sodium silicate as used by potters can be thinned down and used. Sodium silicate comes in different concentrations with 47% being commonly available from pottery suppliers. Thin it down to about 10% and use as for colloidal silica above. .

Repairing brickwork

To repair brickwork, such as when a part of an element retaining lip breaks off, proceed as follows:

• Make sure you have most of the pieces which have broken out. Assemble them together as well as can be done to make sure you have them all.
• Check that they can be fitted back where they belong without great difficulty. Sometimes it may be necessary to remove small lumps or protrusions on the mating faces. Don’t worry, the mortar will fill any gaps.
• Brush away any loose powder on the face of the parent brick and wet it with clean water, to reduce the suction and to prevent too-rapid drying out of the mortar.
• If an element is in the groove take a narrow strip of paper and place it under and up the front of the wire. This will prevent the mortar sticking to the wire. (carefully remove it before the mortar has set hard)
• Apply mortar to the wetted surface using a plasterers small tool (yes there is a tool by that name) or a very small pointing trowel.
• Press the brick pieces into position, applying pressure to squeeze out surplus mortar so that the original line of the brick is restored as far as possible. A slight bulge is OK; it can be sanded off later.

Sometimes it helps to also wet the brick chips, but not too much.

If the applied mortar dries out too fast, remove it and try again. The brick could be wetted a bit more if necessary.

Don’t be too liberal with the water. It all has to be dried out again before the mould or the kiln can be taken to a high temperature.