Single-Fired Ash Glazing – By Richard Aerni

My decision to single fire came about as a result of both personal preference and sheer necessity. The many large pieces produced at Spring Street Pottery in Cincinnati (while working with Mike Frasca and Allan Nairn) would have required significant additional time and money if bisqued. Therefore, the decision was made to single fire as many pieces as possible.

Spray glazing came about because of the obvious difficulties inherent in dipping or pouring glazes on raw pots that weighed up to 75 pounds and could stand as much as 2 ½ feet tall. We discovered along the way that spraying glazes, particularly ash glazes, gave us a control over the final look and a subtlety of effect that we could not easily obtain through other glazing methods. Despite these benefits, we still ended up bisquing a fair number of pieces in the electric kiln, especially containers and closed forms, just to minimize the risk of failure.

Upon moving to a rural region in western New York, I found that my new studio/household had enough electrical power to run those appliances and machines necessary to make pots and to make us comfortable, but no more. I had enough in my bank account to make the studio operational, build a propane kiln, buy some clay, but no more. Not being inclined to fill the 70-cubic foot kiln with pots for a bisque, I opted to single fire exclusively.

Glazing takes place within a home-made spray booth located in a pole barn, which also houses the kiln and my packing area. A box built around an old electric potting wheel gives me a 4 x 4 x 3 foot spraying area, with exhaust provided by an old furnace blower and motor. I spray glazes with a commercial sandblaster equipped with an orifice capable of expanding up to a 1/8-inch opening, and powered by a compressor set a 60 pounds-per-square-inch pressure. Airbrushes and paint sprayers have proven unsatisfactory for this job, as they often clog or their control pins are worn down by abrasive glaze.

Currently, my ash is a mix of 2/3 ironwood (which has excellent fluidity) and 1/3 red oak (which has more refractory properties). The ash is sifted dry through a 40 mesh screen, then sieved when in the wet glaze form through a 100 mesh screen. It is not washed. (Caution: When working with unwashed ash—a caustic substance—always wear a suitable mask and plastic gloves.) The glazes are prepared in quantities sufficient for that day’s glazing only, as they tend to flocculate over time, leading to loss of control of the glazing process.

When glazing, I tend to spray the backside of bowls and platters first, then flip them over and, using a sponge brush, glaze the rims (generally with one of the accent glazes). This “frames” the piece. After the central area is glazed, I apply a succession of slips and other wood-ash glaze accents, sometimes covering much of the surface, sometimes just a trace. I generally have up to a dozen such ash glazes and slips mixed, and just follow my intuition as to how much of which to apply when. This tends to encourage spontaneity, diversity and a greater percentage of those “happy accidents” in the final product.

Relying on glazes that run for a livelihood can be a dicey business. Those that do not run enough are no problem, as they can be refired, with or without additional glaze. However, those that run too much can turn your hair gray. I have learned (usually the hard way) to deal with this in several ways: The first is to apply an “emergency brake” (very refractory) glaze on the lower regions of the pots. The second is to incise a horizontal groove above the foot rims to capture the errant glaze flow. The third, and last resort, is to cut “pancakes” out of insulating brick and place them under the feet of likely suspects as they are loaded into the kiln. Despite all these precautions, wood ash creates so many problems that I am happy anytime I can keep my production of “seconds” below 20% of my total output.

My kiln is a sprung-arch, downdraft propane-fired affair, with a stacking space approaching 70 cubic feet. It is a third-generation descendent of the Minnesota flat-top design. Powered by Alfred-type aspirating burners, it reaches cone 10 in 24 hours. I fire it so that it reaches bisque temperature (cone 08) in 16 hours, with appropriate flat spots along the way to burn off organics, sulfates, etc, and to allow me to catch some sleep.

Turn-off time generally comes in the early afternoon, so I can get into the kiln during the daylight hours of the following day, and get another load firing that same day, if necessary. I glaze in an unheated barn, so during the winter months (which can really stretch on here in upper New York), the heat of the firing kiln is used to advantage in getting the next glaze load together.

Frasca-Aerni Basic Ash glaze Cone 10 reduction
Whiting 12.5%
Wood Ash 50.0%
Ball clay 12.5%
Feldspar 12.5
Flint 12.5%
Total: 100.0

This recipe should be adjusted through testing to properly fit your clay body, and to adjust melt and fluidity to suit your desires. Color variations are achieved by adding oxides; eg, .5-2% chrome oxide, .5-1.5% cobalt carbonate, 1-4% copper oxide, 1-5% red iron oxide, 2-20% rutile, .5-5% titania, etc. Combinations (applied under and over one another) create a wide spectrum of color “flashes.”

A dark leathery glaze variation is used for rims and accents:

Frasca-Aerni Leather Variation Cone 10 reduction
Wood Ash glaze
41.2% Red Iron Oxide 39.2%
Rutile 19.6%
Total: 100.0

Other accent glaze recipes are based on slip clays:

Obsidian Glaze Cone 10 reduction
Albany Slip clay 91.0
Cobalt Oxide 9.0
Total: 100.0

If you are all out of Albany slip, try:

Obsidian Glaze 2
Whiting 6.2%
Frit 3124 (Ferro) 18.7
Sheffield Slip 63.6
Cobalt Oxide 8.8
Bentonite 2.7
Total: 100.0

Sheffield slip is a product of Sheffield Pottery, Post Office Box 399, Sheffield, Massachusetts 01257. This variation is slightly more matte and color response is not as good as with the original Albany slip glaze.

If you are looking for a black gloss glaze with lots of “zippety-doo-da,” try:

Obsidian Glaze 3
Alberta Slip 91.0
Cobalt Oxide 9.0
Total: 100.0
Alberta slip is manufactured by Plainsman Clays, Medicine Hat, Alberta, Canada.

Color Active Slip Cone 10 reduction
Borax 4.5%
Talc 4.5
Whiting 8.9
Feldspar 19.6
Ball Clay 12.5
Bentonite 1.8
Edgar Plastic Kaolin 32.1
Flint 16.1
Total: 100.0

Added oxides further influence color development. Glaze variations sprayed over slip variations often combine chemically to create “ash flashes” of pink, purple, yellow or white, and can encourage crystal growth in the ash runs.

Structural White Stoneware Cone 10
Talc 2.9% G-200
Feldspar 16.1
#6 Tile Clay 23.7
Cedar Heights Goldart 4.6
Hawthorne Clay 7.9
Jackson Clay 17.3
Kaopaque 20 4.4
XX Saggar Clay 5.3
Flint 8.7
Mullite (100 mesh) 9.1
Total: 100.0