Using Wood Ash in Glazes Robin Hopper

Appears in the Jul/Aug 2018 issue of Pottery Making Illustrated.

Wood ash or, more correctly, ash from organic vegetation, has been used as an ingredient for the development of glazes for at least 2000 years. Glazes utilizing wood ash are firmly rooted in Asian ceramic traditions and have been described in many publications dealing with glazes from that region.

Organic ashes come from the burnt remains of trees, bushes, grasses, and even fruits and vegetables. Since their cellular structure gets its mineral sustenance from the soil, it is logical to assume that the residue or ash from the burnt matter also contains those minerals. Different plants absorb different amounts of minerals, and even ashes from the same type of plant taken from different sites or at different times of the year will vary considerably in their chemical content. With these variables in mind, organic ash is likely to contain mineral oxides in the following amounts:

There would almost certainly be various trace elements of other mineral oxides present. These mineral oxides are all materials fundamental to glazes. In fact, all organic ashes will turn to glaze at a temperature between cone 6 and cone 10.

Perhaps the simplest form of ash glaze, other than the natural deposits of ash that occur in a wood-fired kiln, is created by spraying pots with wood ash or painting a pot with glue and rolling it in sieved ash, shaking off the excess and firing to cone 9. At this temperature, the ash will easily melt and the result, to the non-purist, is almost indistinguishable from pottery fired in a wood-firing kiln. I remember visiting a well-known pottery in Japan renowned for its wood-fired pots and seeing a woman swathed in indigo cloth spraying wood ash in a fairly thick coating onto the pots. The pots, I subsequently found out, were going to be fired in an electric kiln!

Making Wood Ash Glazes

There are very simple ways to make glazes using wood ash alone, or ash can be a major or minor ingredient in a recipe. While ash will melt readily at higher temperatures, at low-fire it can only be used as a minor material to give special qualities in conjunction with low-temperature fluxes. Many people have spent a great deal of time studying ash glazes and the processes of collection, burning, analyzing, washing, and using ash. Depending on your personal reasons for making ash glazes, whether for research, refinement, general use, or to develop a surface similar to that of wood-fired ware, you may be either very careful or very lax in the collecting and processing. But there are a number of steps that should be followed to make ash a usable glaze ingredient.

1. Collecting the Raw Material for Burning

Material should be collected in a large volume to ensure that there is enough residual ash to be of use. It takes quite a lot of combustible material to get enough residual ash for even a fairly modest amount of prepared ash. If you want to be able to duplicate your results, make a note of the type of material, site, and date of collection.

2. Burning the Raw Material

The material should be burnt on a dry, clear, windless day, on an area such as a cement slab or cement blocks where there will be as little contamination as possible from other sources. Material burnt in iron grates or garbage bins is likely to become contaminated with, and subsequently colored by, iron scale from the rust invariably present. Light the fire and allow it to burn completely, raking unburnt material to the center of the fire so that as much ember turns to fine gray ash as possible. Collect the fine ash as soon as it’s cool. If it looks as though it might rain before the ash is collected, cover the ash with something non-combustible, as rain will leach some of the minerals from the ash.

3. Sieving the Dry Collected Ash

In order to remove the charcoal and other heavy, partially burnt material from the ash (1, 2), it’s best to first screen it through an ordinary garden sieve of approximately half-inch mesh (3). After this it can be screened through a finer-mesh sieve, until only a fairly fine powder remains. Caution: A dust mask and eye protection should be worn when dry sieving ash; it is a caustic material that can easily damage eyes and lung tissue.

4. Washing the Ash

There are two differing views on whether or not ash should be washed. When it’s washed, various soluble alkaline materials are removed in solution with the washing water, and many feel that they’re throwing away precious trace ingredients and fluxes. The reasons for removing these materials are that they might enter the pores of the clay body and create various problems when they melt, and that they’re caustic. However, this is a personal choice generally best made from the experience of doing it both ways. Personally, I prefer to use unwashed ash with the soluble materials intact.

Whether washing ash or using unwashed ash (4), it’s important to wear rubber gloves. Ash materials can easily cause skin problems from even short contact. When I was new to making ash glazes, I worked at sieving a large amount of ash in water with no protection. After an hour or so, my hands started to itch and a little while later blood started to seep from the pores on the back of my hands. The net result was an inability to work for three weeks while they healed. So adequate precautions are extremely important!

The usual method of washing ash is to put it into a plastic container and cover it with water. After a day, remove floating particles and discard them. Let the ash soak for a week, then pour off and replace the water and soak for another week. Repeat this process until the water is clear and has lost any soapy feeling. By this time there should be little or no soluble caustic material left, so there should be no risk from them.

5. Drying the Ash

After washing is complete, siphon off the water and dry out the remaining sludge in bisque-fired bowls or plaster drying bowls. When dry, sieve it through a 60- or 80-mesh screen (5) and store it for use.

Using Ash as a Glaze Ingredient

The process of washing, drying, and sieving is generally done by people who are being very careful in the preparation of the ash they use. However, it’s not strictly necessary if the ash is not going to be washed for the removal of soluble alkalies. In this case, the collected ash, from whatever source, can be merely sieved through a 30-mesh screen and used directly. Ash used in this way will probably be somewhat gritty but it usually smooths out in the firing and creates different textures from ash that is finely prepared. It’s important to remember to wear rubber gloves when using glazes containing unwashed ash.

Remember that ash is a highly variable material, so collect enough at one time for a reasonable future supply. It can be frustrating when you have developed a really nice glaze and the source of the ash disappears. For several years, I used a glaze I made with equal parts ball clay and ash from the fireplace of a local pub, consisting mainly of the ash from the sawdust used to soak up the spilled beer and whatever other sundry material was swept up. I moved away from that location and never did find another comparable supply. The beautiful pale gray-green glaze, with yellow-green fluid markings, was never the same again.

Since ash is a complex material, which, on its own is capable of producing very interesting effects, it follows that simple additions of other materials, to add to the melting or to add color and texture, may be all that’s needed for the production of subtle and interesting glazes.

It’s quite simple to develop glazes using ash as a major ingredient by line blend, triaxial, and quadraxial methods. Because of its infinite variation, ash is always an exciting material to use and responds to glaze coloration in interesting ways. Here are a few possible mixtures for ash glazes that will give a wide variety of surfaces and color potential at the higher temperature ranges.

For glazes in the low-fire range, the use of ash is more limited due to its lack of fusion at lower temperatures. However, it can be used with great success in glazes where there is a good deal of other fluxing material, and can provide a very interesting subject for study and use. Here are a few suggestions for glazes developed with ash at low-fire range.

Much of the quality of ash glazes lies in their rather individual and changeable nature, fluid quality, and intriguing effects on color and surface, mainly from the all-important trace elements, particularly phosphorous. Exciting surface effects can also be developed using the natural fusion, at high temperatures, of organic materials applied directly to the surface of the clay. Techniques of wrapping pots in brine-soaked straw have long been used by the Bizen potters of Japan. The combination of salt and ash leaves calligraphic markings on the clay. Most organic materials will leave enough ash to fuse and make flashings of color and glaze. They don’t need to be soaked in salt to fuse. Similar fuming effects can be obtained by placing pieces of bone on the unglazed ware and firing to cone 8–10. The bones will shrink considerably, usually leaving tracings and fumings from their calcination.

Fake or Simulated Wood-Ash Glazes

From the list of minerals that make up an ash it’s easy enough to see that a basic glaze can be developed with much of the same physical qualities as a natural-ash glaze. The fluid linear markings, rivulets, and islands of calcium raised above a flat, sometimes matte and sometimes shiny and crackled lower area, are typical of ash glazes. The key components for making simulated ash glazes are high concentrations of calcium and other alkaline earths mixed with fusible clays such as Alberta slip clay or ball clays along with some feldspar. They are generally fired in reduction between cones 6 and 10. Simulated ash glazes respond well to being over-sprayed with colorants. Since they usually have at least 20% clay, they can be used for once-fired glazes.

The following bases give starting points for developing simulated ash glazes:

Glazes for Once Firing

Glazes for once-firing, or raw-glazing as it’s sometimes called, can provide the ceramic artist with certain economical advantages in both fuel costs and in the time spent in handling the wares. Most important, it can be done without detrimental effect to either the aesthetic or functional quality of the ware.

There are two reasons why once-firing is not all that common. One is that the work has not previously been bisque fired, so certain difficulties in the handling process become evident. The other is that in order to be sure the glaze adheres satisfactorily to the body and will shrink at the same rate as the body, the glaze normally has to have a fairly large amount of clay in it. Although the high clay content undoubtedly has the effect of limiting some of the glaze palette, once-fired glazes may be colored in all the usual ways.

There are many who prefer the once-firing process because of its immediacy. The work is fresh in the maker’s mind, so a more spontaneous result develops. For others, many surface decoration techniques such as carving, sgraffito, some slipware techniques, and glaze trailing seem to work particularly well with it. Once-fired glazes are also good in conjunction with the vapor glazing processes of salt and soda firing.

What makes a good starting point for once-firing glazes? Basically, any glaze that has a clay content of 20% or more will probably work quite well as a once-firing glaze. Many naturally occurring clays such as Albany and surface clays found close to streams and rivers can be used with few or no additions as once-firing clays at high temperatures. Many of the glaze bases suggested throughout this book that contain a fairly high percentage of clay work well when used in single firing.

Problems arise with the handling of the object when wet, and with the clay of the object itself, which may not be particularly amenable to absorbing fairly large amounts of water. It will be necessary to find a clay with enough green strength to neither crack nor distort when it is coated with glaze. Some clays are best glazed when bone dry and others when the ware is in a leather-hard condition. The only way to find the solution to all these questions is by trial and error.

The best pointers to the successful development of once-fired glazes, for use at any temperature, are:

1. Use a clay body that hasn’t been opened with more than 10% grog and preferably contains at least 20% ball clay.

2. Use a glaze base that contains at least 20% clay.

3. Mix the glazes with as little water as possible, even to the point of adding a small amount of deflocculant such as sodium silicate (1% maximum of the overall weight or 0.2% of the clay content—more may cause a gel to develop).

4. Don’t apply too thickly. The glaze might flake off, leaving bald patches on the work and glaze on the kiln shelf.

This text was excerpted from Robin Hopper’s book The Ceramic Spectrum, which is available online at the Ceramic Arts Network Shop.