Working with Ash in Glazes Alisa Liskin Clausen

Appears in the March 2018 issue of Ceramics Monthly.

Ash glazes have been used for over 2000 years. Potters who stoked their long-burning kilns with wood noticed that after many firings, the interior kiln bricks glistened. The ash from the wood was coating the bricks, adhering and melting as a thin layer of glaze. Putting one and one together, the potters discovered that wood ash, with very little else added, forms a glaze.

Traditionally, ash glazes were fired between cones 9 and 10. Electric cone 6 glazes became popular as urban real estate prices rose, pushing potters into smaller spaces, which virtually only allowed for electrical sockets and electricity to fuel their kilns. That led to many potters altering their cone 10 glazes to melt at the lower cone 6 temperature in their small, but practical studios. Ash glazes, on the other hand, seemed out of reach because they required big outdoor kilns and long days of hard labor firing; however, many potters researched making ash glazes at cone 6, finding ways to create ash glazes that are useful for electric kiln firings.

Each year in Scandinavia, after the popular midsummer bonfires smolder and go out, there is a wealth of wood ash to be collected for making ash glazes. I had never made an ash glaze; however, with access to so much free glaze material, I was giddy with curiosity. It would be a waste not to investigate what the wood ash could contribute, either by making an ash glaze from scratch, or just as interesting, to add a percentage of ash to a glaze I already use. It requires the same work I usually do for any glazes test trials and up to half the material is free.

I read about ash glazes and looked at a lot of recipes for both natural ash and fake ash glazes, ranging from cone 6 to cone 10. Calcium was the constant oxide in both ash and fake ash recipes. I spoke to a friend who fires a wood kiln to compare notes and learned more about ash glazes in general. Then, I made some educated-guess tests, using a high percentage of ash and added materials to help the glaze melt closer to 2228°F (1220°C) than 2372°F (1300°C).

What Does Wood Ash Contain?

Wood ash contains high amounts of calcium and low amounts of alumina and silica. However, the silica and alumina in the clay body can fuse with a thin layer of ash glaze to make a glaze that’s harder than imaginable without much of those ingredients needing to be  directly added to the glaze. Wood ash is variable in composition, so it’s not a glaze constituent a potter can depend on from batch to batch. Every single time wood ash is collected, even if it is from the same wood of the same tree, each batch will differ from the last. A lab analysis would be necessary for each batch of ash, so that the potter could alter each one of them with oxides, in order for each batch to be as identical as possible. I prefer to work with the variable nature of the gathered ash and see what happens from firing to firing. Ash can be added unwashed or washed to a glaze mix as a dry ingredient. It can also be mixed with water to make a wash, and sprayed or brushed over a glaze.

The advantage of using washed ash is the removal of soluble sodium/salt. Since ash is a variable material, we can at least remove the soluble sodium. Without a soluble material in the glaze slurry, a batch of glaze will be more consistent as time lapses between each firing of that particular bucket of glaze. However, it is more labor intensive than using unwashed ash.

Ash can be used unwashed as well. Take out all of the big pieces of charcoal, then mash and sieve the remaining ash. Leaving in the big pieces might add texture, but it might also just leave dry patches of material that won’t fully melt. Remember to wear a properly fitted respirator with dust filters when working with dry ash. It can be very irritating to your airways and throat.

Preparing Ash

Prepare enough washed or unwashed ash so that all of your tests are made from the same batch. To be completely consistent, make a big batch of ash, so you can reproduce bigger glaze batches of your favorite glaze tests. At some point you will run out of the batch, but the same species of washed wood ash will yield similar results.

The ash used in these tests was washed, sieved, mashed, and dried. Wet ash is caustic to the skin so wear rubber gloves when washing the ash and when using ash glazes. To prepare the ash:

1. Let the ash sit in a bucket of water at least overnight.
2. Sieve off the rinse water, and repeat. The water may be yellowish the first two times or so. Repeat the process until the water sieved off of the ash is clear, which usually happens after three or four washes.
3. Remove all the big pieces of charcoal and other debris, which either float or sink.
4. Sieve, mash, and spread the wet ash out on a plaster surface and let it dry (1). It can take days to dry, as the sieved ash is dense and almost cement like in consistency.
5. Once the ash is dry, wearing a respirator and working in a well-ventilated area, pulverize the dried fine ash.
6. Now, that the ash is prepared, weigh it in to the glaze mix like any other ingredient. When ash in this finished form is added to a glaze, it is sticky and clogs up the sieve. To save mixing and cleaning time, I add it to the glaze slurry last.

I found that a lot of wood makes very little ash. It can take quite a while to collect and prepare a couple of kilograms of ash but it is well worth it.

Making Ash Glazes From Scratch

Reading through different references on ash and ash glazes, I learned that the three hallmark characteristics of the surface are rivulets (also called stringing), flashing, and fluidity. Glazes mixed with large amounts of ash and not much clay have low viscosity and it can be difficult to get them to adhere to vertical surfaces because they run so much. Adding more clay can fix that and make the slurry easier to glaze with, but adding too much clay will make the glaze too stiff, actually more like a slip glaze. I found that adding red clay made the ash-dominated glazes adhere well, while also adding iron for color.

Ash acts as a flux, but because it contains so much calcium, which starts to flux at around 2012°F (1100°C), there is not much time for the flux to help melt the glaze before the kiln reaches  cone 6 (2228°F (1220°C)). Additional flux(es) have to be added to lower that melting point. An ash glaze for cone 10 can be ash and clay. At cone 6, after reading more on ash glazes, fake ash glazes (which can contain a lot of whiting in order to add enough calcium), desired ash glaze effects, coloring, and potential fluxes, I used the following three glaze ingredients for a triaxial blend (2):

A=Wood Ash B=Red Clay C=Nepheline Syenite

Ash sometimes creates more than one eutectic melt, which can be seen in triaxial blend tests. In my results (3), there is a clear eutectic melt diagonally from top to bottom from tile 2 to tile 20; these are the glazes made up of higher proportions of ash or nepheline syenite.

A line blend of similar materials (A: ash and B: an equal parts mixture of nepheline syenite, red clay (Oldenwalder), and JM Frit 169 (a boron frit), shows from 0% to 100% of ash and the frit/nepheline syenite/ red clay mix. (4). Every tile from 1 to 10 is melted, with 1–4 being less glossy. The increased amount of red clay in the tiles 5–10 increased the color in the glaze surface. The last tile (10, 100% of the B mixture), is melted, but with no addition of ash, it is simply a slip glaze with no ash characteristics. Tile A (100% ash) is melted but with a rough center where the ash was layered thickly. The sides and edge of the glaze ring are smooth and melted, with a slight flashing at the edge of the glazed area.

Adding Ash to a Known Glaze Recipe

The next set of tests show how ash can be added to any known recipe for new surface effects. The first test is a slip glaze recipe (Zakin Mouse). I added 50% ash to the dry mix (5). Left tile: Zakin Mouse glaze (85% Albany slip (substituted Oldenwalder red clay), 5% nepheline syenite, and 10% wollastonite). Right tile: Zakin Mouse glaze with an additional 50% ash, 1% cobalt oxide, and 3% rutile added for color.

From a recent set of glaze tests I had done (listed at the end of this article) from John Britt’s book, The Complete Guide to Mid-Range Glazes, I still had the original test batches in cups. I revised the tests twice, first by adding 1% cobalt oxide to each cup and firing new tests, then doing an additional set of tests with an added 3% rutile (6). Finally, a third set of tests was made with the addition of 50% ash to each test cup (7). Each test batch of 100 grams yielded 4 tests. Upcycling glaze tests in this way, and using waste materials such as ash make a conscious contribution to lessening the potter’s carbon footprint.

To account for the loss of material for each successive test, when adding materials to wet test glazes, weigh the wet test batches and then add the appropriate percent of the additional material. For example, if my test cup had a tare weight of 75 grams before adding the ash and I wanted to add 50% ash: I calculate that  50% of 75 = 37.5, and add 37.5 grams of ash to the wet test. While not exact, it is a good way to get quick reproducible results that can be refined later.

Results: Rivulets/Stringing and Pooling

The addition of ash made these surfaces exciting, several with the characteristic rivulet/stringing effect on the surface. I tested them all, both in bowls and on upright tiles (8) to observe the glaze’s movement. None of the tests ran. The bowls had a thicker application than the tiles, and some of them pooled. But, with one dip and a second quick half dip, the upright tiles did not run either. Caution: Always protect your kiln shelves when testing ash glazes; they can be very runny. Remember that your glazes and firing times will be different than mine.

After making these tests, I’ve decided to add ash as one of my regularly used glaze mixing materials. It is an exciting material because it is so variable. It also a plus that by using ash we are recycling a free by-product of burnt wood, which otherwise may be just wasted.

Recipes and Results

The next three tests (9, 10, 11) highlight the glazes without ash and then with the addition of ash. The rivulets and flashing from the added ash is a surface that potters who fire to cone 6 can easily achieve with some consistent tests of a very inconsistent material.

the author Alisa Liskin Clausen is American born and lives in Southern Denmark. She has a BFA in ceramics from Syracuse University, and has concentrated on cone 6 oxidation glazes since 2000. Her extensive glaze tests are shared on several ceramic forums and databases including the Sankey database, Glazy.org, and Flickr.