Battwash

Battwash explained

If taken to a sufficiently high temperature, glass will reproduce the  fine detail and imperfections of the supporting surface. It’s also likely to bond to those surfaces.

Among the first cautions received by newcomers to kilnforming is to “be sure and battwash your hearth, kiln shelves and moulds”. Lots of names other than battwash may be used, including shelf wash, shelf primer, shelf paint, parting agent, release agent, separator.

Whatever the name, they are a mixture of fine powders mixed usually in water. When the water dries out a coating of tightly packed powders remain. The powders chosen are called refractory powders as they are the ones best able to withstand high temperatures.

Applying it as a ‘paint’ makes it easier to obtain a dense layer on regular or irregular shapes and on near vertical surfaces.

The purpose of the coating is to allow individual particles of powder to leave the mould or other supporting surface should there be bonding between the particles and the glass. The mould or shelf, and the glass itself, will be undamaged. It’s something like greasing a cake pan.

It should be understood that glass will sometimes bond to battwash. The important thing is that the glass doesn’t bond to the mould or shelf to which the battwash has been applied. With few exceptions, all forms moulds and shelf surfaces which may be contacted by glass require a battwash.

A silica free material such as alumina is the best powder to use, but wet alumina powders are difficult to spread evenly, becoming very ‘ropey’ and uneven in texture. The addition of a clay-like material such as kaolin makes the wet mix much more spreadable, as well as improving the adhesion to the coated surface. Only as much kaolin should be added as is necessary to make a brushable mix.

A recipe

A most effective mix, measured by volume (in a calibrated container) is:

• 60% Alumina hydrate or calcined alumina, and
• 40% kaolin
• mixed in water to a consistency suitable for the surface being coated.

This can vary from six parts of water to one of the powder mix for non absorbent surfaces to 12 parts of water for absorbent materials such as kiln shelves and fibreboard. An organic colouring material such as a food dye is sometimes added to make it easier to see where a coat has been applied.

The addition of about 2% bentonite (another type of clay) improves brushability even further. Bentonite changes colour between wet and dry, eliminating the need for the colouring agent in the battwash. If adding bentonite, the mix should be left for 24 hours after the water is added to allow the bentonite to fully swell. All the ingredients can be obtained from pottery supply outlets. Buy the smallest particle size powders available to achieve the smoothest surface. The dry mix has indefinite shelf life whilst the wet mix may get a bit smelly after some months, depending on the weather and how clean was the water used.

The wet mix settles out rapidly so needs to be continually stirred for consistent results. There’s no need to have different thick and thin mixes: one can do both jobs. With a container of wet mix which has settled, use the brush to stir up only a small amount of the powder and apply that as a thin mix: stir more thoroughly if a thicker mix is desired. 

There are premixed coatings available, sold mainly through pottery or art glass suppliers. Many have been imported to Australia, so may have strange names. They may also have been formulated for use at pottery temperatures and may not function properly at glass temperatures. There have been costly failures due to the use of unsuitable premixed battwash. Even those sold through glass outlets can be suspect. Always test any of these mixes with scrap glass before committing good glass.

Much has been written on the techniques of applying and maintaining battwash coatings, particularly in pottery books. Much of it is ill conceived.

How to apply battwash

As a battwash, the coating need be only thick enough to prevent contact between glass and base support. Should glass stick to the battwash it will be to the topmost layer of powder. Having 20 or 100 more layers underneath doesn’t make it any more effective. Generally, a thicker coating is more likely to crack than a thinner one.

A few pointers:

• Make sure that the surface to which the battwash is to be applied is as smooth as possible. On kiln shelves, lumps and minor eruptions can be removed by sanding with a fine abrasive paper on a sanding block. 100mm wide 150grit or 180grit sold-by-the-metre abrasive paper are good for this sort of work.
  

• It is not necessary to build up thick layers of battwash. A few thinly applied coats is usually enough.
• Battwash settles out quickly. Stir the mix constantly.
• Vary the consistency of the mix to suit the absorbency of the material being coated. One can do this by varying the frequency of stirring and taking a thinner mix from the top of the container when coating more absorbent materials.
• Try pre-wetting fibreboard or other highly absorbent materials.
• Brush succeeding coats in different directions.
• Brush lightly. Brushing too heavily can remove much of what was applied in a previous coat.
• Make sure each coat is dry before applying the next.
• A wide, reasonable quality paint brush is adequate for applying the coatings. It is NOT necessary to use ‘hake’ or other expensive brushes.
• Gentle smoothing with a fine soft cloth will remove brush marks or fine surface imperfections on broad surfaces. Use a finger or the heel of a hand on more difficult shapes. Do this after the coating has dried.
• Fine abrasive paper can also be used  to remove high spots if battwash has gone on too ropey.
• It is NOT necessary to remove all the battwash after each firing.
• Gentle rubbing with a soft cloth will disturb the surface layer of powder and prepare the coating for the next firing.
• Don’t try and patch up chipped or crazed battwash with new. Better to get rid of any loose or chipped material; rub to an even surface and then re-coat. Medium grade abrasive paper is OK for this.
• Used battwash can be removed from uneven surfaces or irregular shapes by brushing. Always use a brush softer than the surface to be cleaned. One must be particularly gentle with fibreboard or vermiculite (Ceramaguard).

 Note. The whitish patch near the centre of the batt below is where a depressed makers brand has been filled so that both sides can be used. Dry battwash powder mixed with colloidal silica instead of water was used.

Effectiveness & life span of battwash

Testing has shown that the likelihood of battwash sticking to glass increases with each extra firing on undisturbed battwash, and that disturbance or removal of the surface layer of particles is all that is needed to restore the battwash. Of course, each rubbing makes the coating a little bit thinner, so re-coat as required. In one series of tests a number of pieces of float glass were fired to 780°C on a battwashed shelf without disturbing the battwash surface between firings.

There was no pick-up after the first firing, slight pick-up after the second, more after the third and much more after the fourth. After the fourth firing the shelf was rubbed gently with a soft cloth to disturb the surface layer. There was no pick-up from two subsequent firings after the shelf was lightly rubbed between firings. 

The extent to which battwash will adhere to a piece of glass varies with the firing temperature; the hotter the glass the greater the possibility of pick-up. Using the word in a most unscientific sense, the more ‘sticky’ the glass the more the pick-up. Some of the leadlighting glasses have lower working temperatures than does float or even some of the certified products. If these are fired at temperatures appropriate to float there can be a thick build-up which is difficult or impossible to remove.

Thicker battwash mixtures can be used to create texture or pattern by brushing when wet or by extruding through an icing tube or from a sauce bottle. A thick layer of tightly packed powder will remain when the liquid has evaporated. Thick deposits are more likely to crack and should be removed in their entirety after being fired.

ALWAYS make sure that moulds shelves or other battwashed surfaces are completely dry before firing. If in doubt, dry in kiln at between 100°C and 150°C (215°F and 300°F).