Alternate Names: Pumicite
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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).
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.
Out Bound Links
In ceramic technology the term 'glass' is contrasted with the crystalline state, it is seen as a "super-cooled liquid". When crystalline materials solidify the molecules have opportunity to orient themselves in the preferred pattern during freezing whereas in a glass the random orientation of molecu...
In Bound Links
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
Joseph Herbert overviews the technical and practical aspects of this interesting group of materials
Mount Saint Helen's Ash
Mount Pinatubo Volcanic Ash
A glaze that employs organic ash (e.g. paper, wood) as a supplier of oxides (e.g. silica, alumina, soda, calcia). Ashes are generally flaky and fibrous and cannot be screened well, this consistency contributes to variegation in the color and texture of the fired surface. Many pottery books deal with...