Matt’s Volcanic Palm Ash

Matt's Volcanic Palm Ash
Matt’s Volcanic Palm Ash

Washed Palm Frond Ash 60m – 45

Mt. St Helen’s Ash – 30

Custer Feldspar – 15

Redart Earthenware – 10

Volcanic Ash

For a while now I’ve been interested in experimenting with the possibilities of Volcanic Ash. My last post alluded to my research into the possibilities of high silica ash that I’ve been processing myself. Like most of my testing, I like to cast a pretty wide net of different potential recipes and gradually zero in on interesting effects and variations. In the process of varying the types of clay in my West Palm Ash recipe, I also experimented with the proportion of ash to feldspar and remembered that in some cases Volcanic Ash can be a 1:1 substitute to feldspar. On a whim I chose to finally test some Volcanic Ash in my recipe. The results were pretty stunning!  Having thrown a bunch of darts now and nailing the bullseye, my next round of tests will further explore the potential for this material! Stay tuned, as I’ll be posting all of my tiles from the last few firings!

Ash Column from Mt. St. Helens

A picture of the eruption of Mount Saint Helens in 88′. This is where the stuff comes from, and it’s easy to see while the material itself isn’t technically an ash in the traditional sense of the word, the name sticks. The following pictures are closer images of what this stuff looks like under microscopes.

Volcanic Ash

 The foamy, almost bubbly texture of pumice happens because of simultaneous rapid cooling and rapid depressurization. The depressurization creates bubbles by lowering the solubility of gases including water and CO2 that are dissolved in the lava. This process causes the gases to rapidly exsolve. It’s like the thing that happens when you open a can of Dr. Pepper! Add that process to the simultaneous cooling and depressurization , and you have ash or Pumice with an interesting bubble matrix!


Dolomite with Magnesite
Dolomite with Magnesite

From Wikipedia:

Dolomite (pron.: /ˈdɒləmaɪt/) is a carbonate mineral composed of calcium magnesium carbonate CaMg(CO3)2.

Dolostone (dolomite rock) is composed predominantly of the mineral dolomite with a stoichiometric ratio of 50% or greater content of magnesium replacing calcium, often as a result of diagenesis.

Dolomite is used as an ornamental stone, a concrete aggregate, a source of magnesium oxide and in the Pidgeon process for the production of magnesium. It is an important petroleum reservoir rock, and serves as the host rock for large strata-bound Mississippi Valley-Type (MVT) ore deposits of base metals such as lead, zinc, and copper. Where calcite limestone is uncommon or too costly, dolomite is sometimes used in its place as a flux for the smelting of iron and steel. Large quantities of processed dolomite are used in the production of float glass.

In horticulture, dolomite and dolomitic limestone are added to soils and soilless potting mixes to lower their acidity and as a magnesium source. Home and container gardening are common examples of this use.

Dolomite is also used as the substrate in marine (saltwater) aquariums to help buffer changes in pH of the water.

Particle physics researchers prefer to build particle detectors under layers of dolomite to enable the detectors to detect the highest possible number of exotic particles. Because dolomite contains relatively minor quantities of radioactive materials, it can insulate against interference from cosmic rays without adding to background radiation levels.[14]


Dolomite can be used in glazes melting over 1170C to produce a silky matte surface. This occurs because high percentages of dolomite help to form diopside crystals (CaMg(SiO3)2) on cooling, and it is these that produce the popular butter-matte effect. This effect is most pronounced in reduction.

Dolomite as a ceramic material is a uniform calcium magnesium carbonate. In ceramic glazes it is used as a source of magnesia and calcia. Other than talc, dolomite is the principle source of MgO in high temperature raw glazes. ‘Dolomite matte’ stoneware glazes, for example, are highly prized for their pleasant ‘silky’ surface texture. Dolomite by itself is refractory, but when combined with the typical oxides in a glaze (especially boron) it readily enters the melt.

Commercial dolomites are not able to achieve the theoretical 54:46 calcium carbonate:magnesium carbonate ratio, they tend to have less magnesia. It is simple to do an LOI test by firing a specimen of powder in a thin bisqued bowl to confirm the consistency of dolomite shipments. The chemistry shown here is theoretical and many commercial materials approach this with much less than 1% of two or three other oxides (e.g. Al2O3, SiO2).

Dolomite is a carbonate (like whiting) in that it loses considerable weight during firing when it disassociates to form MgO, CaO and CO2, this process being complete by about 900C. However, in many circumstances where a raw glaze employs both CaO and MgO, dolomite is an economic alternative to sourcing with a mix of calcium carbonate and talc. However care needs to be taken to obtain a consistent grade since dolomites tend to vary more in mineralogy and can contain iron contamination that can darken the fired glaze.

Synthetic substitutes to source MgO and CaO (e.g. frits) are worth considering, especially if glazes are not high temperature. Frits have no loss on ignition (therefore do not generate glaze bubbles) and melt far earlier than mineral sources of MgO and CaO. Using ceramic calculations it is quite easy to adjust a recipe to source MgO from a frit instead of raw materials.

Soda Fired Tenmoku Glazes

Soda Fired Tenmoku Glazes

Shop glazes from the Armory Art glaze lab.

Sprayed 1st layer of glaze blend (75%Angel Eyes/25%Tenmoku)
Sprayed 2nd layer of Strontium Crystal Magic using a large droplet spray pattern, and a very light application.
Sprayed 3rd layer of glaze blend ( 33% Tenmoku/33%Angel Eyes/33%No Man’s Black) Sprayed very heavily in and out.

Fired to c10/11 in Soda kiln, with 333g borax/333g soda ash/333g baking soda -> 1 gal Hot H20 introduced via garden sprayer. (Notice the badass blue/green glass in the bottom? That’s the soda!)