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Volcanic Ash

Alternate Names: Pumicite

Oxide Analysis Formula
CaO 6.00% 0.42
K2O 1.00% 0.04
Na2O 4.00% 0.25
MgO 3.00% 0.29
Al2O3 15.00% 0.57
SiO2 67.00% 4.35
Fe2O3 2.00% 0.05
TiO2 0.50% 0.02
LOI 1.00%n/a
Oxide Weight 383.98
Formula Weight 387.86


Pumicite is composed of minute particles of volcanic glass. It is easily processed due to its fine in nature. Deposits result from volcanic eruption blowing large quantities of material into the air which settles in layers, sometimes in thicknesses that compress to 30 feet or more! Volcanic ash particles are non-crystalline due to the quick cooling during formation. This material thus has the melting properties associated with a glass or frit (rather than melting at a specific temperature, it softens over a range of temperatures).

The chemistry is sometimes similar to granite or ryolite. Raw color varies and deposits can be found over wide areas of the central and western areas of North America (and world wide). Volcanic ash materials can vary widely in their chemistry, so it is difficult to propose a representative analysis.

Some types of volcanic ash are balanced enough chemically that they can be used in amounts to 50% or more of the glaze batch. Most require the addition of fluxes and kaolin (to supply Al2O3) to form a glaze with reasonable firing and suspension properties.

Since ash chemistries vary so much it is not practical to expect that if you satisfy the ash percentage for a glaze recipe you find online that it will work as expected. The best formulation method is to accumulate or buy a sizeable batch of ash and get the chemical analysis (or have one done). Then use glaze chemistry calculations to compare the chemistry of the ash with a target formula for the intended temperature. Then add the oxides the ash is lacking to make it into a glaze. In this way the maximum amount of ash can be employed in the recipe (often achieving the most interesting visual effects).

Related Information

Comparison of three volcanic ash chemistries

Tap picture for full size and resolution

Notice the amazing similarity between Mount St. Helen's and Mount Pinatubo ashes. And they are from opposite sides of the planet! Yet a common commercial material in North America, Navajo Pumice, is completely different.


Articles Formulating Ash and Native-Material Glazes
How to have a volcanic ash analysed and them use ceramic chemistry to create a glaze that contains the maximum possible amount of the ash for the desired effect
Articles Volcanic Ash
Joseph Herbert overviews the technical and practical aspects of this interesting group of materials
Glossary Wood Ash Glaze
Common washed wood ash has a chemistry akin to a ceramic glaze, so it can comprise significant percentages in a recipe. Plus it can produce unique visual effects.
Glossary Glass vs. Crystalline
In ceramics, understanding the difference between what a glass and crystal are provides the basis for understanding the physical presence of glazes and clay bodies.
Materials Volcanic Ash 2
Materials Andesitic Pumice
Materials Navajo Pumice
Materials Mt. St. Helens Ash
Materials Mount Pinatubo Ash
Volcanic ash beds of North Dakota
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes Flux Source
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
By Tony Hansen
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