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On Ash, e

One good look at data on the ash of different wood and plant species shows that treating them as a group has limited functionality. Some are high in magnesium, others calcium and still others potassium. Further the amounts of silica and phosporous vary from undetectable to significant amounts. Further, where they are grown, when they are harvested and how they are burnt, and how long they are stored has an impact on elemental composition and their rhelogical properties. Consequently the one generaliation about ash that makes sense is that there are no good overall generalities about plant ash. This short piece will lay out things that seem to cause variation that I know about, things I believe but am reasnably certain of, and things that I think are likely but have less certainty of. If you dislike qualificatins you should stp reading now.

When I started ceramics I learned about only a few kinds of ash, softwood, hardwood, and fruitwood ash, and then we only had generalizations about them. The main advice was to try them and see.

In Ceramics we use the phrases "soluble", "insoluble", "slightly soluble" , and "very soluble" in many different ways. When talking about fired glazes, especially about lead, it takes very little solublity to be called "highly or very soluble". But in terms of use of materials in glaze slurries, once the amount of dry material that can go into solution even approaches 1/2 of 1% it is really "very" soluble.

All soluble salts affect the rhelogy, the fluid properties, of a glaze slurry. As time has passed in my career as a ceramic artist, understanding and controlleing these properties has becme more and more important. Consequently, terminlogy pertaining to solutions and rheology has become critical to understand.

  • Specific Gravity of a slurry,
  • Solution
  • Suspension
  • Slurry
  • flocculation
  • deflocculation
  • thixotropy
  • crystaline
  • amorphous
  • sintered
  • fused

Further, because solutions are rather ideal circumstance for reactions between materials to take place, having an understanding of reactions that can take place is helpful.

There are three categories of base metals we will talk about, the Alkali Metals Sodium and Potassium, The Alkaline Earths, Calcium and Magnesium and everything else. However, the "Everything Else" category is really a group of materials that do not replace each other well. They have so many unique properites in glazes that they don't lumb together well.

So to try to boil this down to individual topics to be disucussed I end up with this:

  1. How ash differntiates itself by species say hardwoods, fruitwoods, grasses, bean family trees, or by parts of a plant say rice stalks versus rice hulls.
  2. How ash varies by the age of the plant, how it is handled after it is dead.
  3. How ash varies by how it is burnt, stored or washed.
  4. How ash in atmospheric kilns deposits on pottery..
  5. Factors affecting glaze slurry rheology..


  1. Species
    1. American Softwoods, pines, juniper, spruce.
    2. American Hardwoods, Cottonwood, walnut, ash wood, poplar
    3. "Fruit woods"
    4. The beans, locust, tamarind, mequite, Texas ebony,
    5. Grasses, the grains such as rice, corn, bamboo, sugar cane, palm,
    6. Eucalyptus family,,, "gum trees"
  2. Age
    1. leaching of soluble salts as a plant ages, or is dead standing, or left in the rain. The likely effect of potassium retention is thicker wood left in the rain. MAybe? something about leafs an stems if there is enough data available.
    2. bagasse ash,
    3. hulls of grains.
  3. how things are burnt,
    1. melting points
    2. volitization
    3. sintering and fusing, likely temperatures and effects on solubilities
  4. Ash differntiation in wood kilns, mention of paper on loss of potassium and sodium based on temperature.
    • In order to talk about melting, fluxes, eutectics, sintering, we need some common vocabulary. Often numerous definitions of things exist, some properties can be conceptualized in numerous ways. Words attempt to describe reality, to organize how we think about it and are necessarily simplifications.
    • Normally we talk about a glaze melting, and yes, the ingredients go from being solids to being liquids, but once we have some liquid, it is easier to conceptualize the melt in terms of solutions.

Important terms:

  1. A liquid solution is,, solid solution
  2. Solvent

A solvent is a chemical that dissolves another chemical. Water dissolves sugar in syrup. Water is the solvent. Water is the solvent for salt in sea water. However, there can be solid solutions where a solid holds another solid in solution.

  1. solute
  2. a saturated solution
  3. Super cooled solution
  4. devtrification
  5. seed crystal
  6. phase diagram
  7. liquidus
  8. eutectic, relative eutectic

There is a stereotypical tendancy among Jews to define one word in terms of a related word putting the two words into opposition. The classic definition is of shlameil and schlamozzle. The schlameil spills the coffee on the schlamozzle. That is the klutz spills the coffee on the unlucky. In fact, all definitions put words opposed to other words. To generalize, a rock is not a non-rock. Fast is not slow, light is not dark, a car is not a bicycle. Definitions put a box around a concept and separates it from other concepts. I see this idea in Taoist or Buddhist terms. The definition is not reality but like a map is to the city. There are lots of analogies ways of explaining things. In terms of this essay The solvent is the liquid or solid that the solute dissolves in. Often, especially when oe of these is liquid at the temperature you are talking about and the other is solid, this is obvious. The ocean (solvent) dissolves the salt (solute). No one would think that the salt dissolves the liquid ocean. But if you add water to pure alcohol is the alcohol dissolving the water or is it the other way around? With ice and salt which is melting which? But if you put frozen water with frozen alcohol it becomes less clear. This makes it a good example. Water freezes at 0 alcohol at ??. But if you mix a little water in the alcohol the freezing temp decreases. In old clayer terms the water fluxes the alchohol. These days fluxes are being defined as belonging to the first column of the unity formula chart rather than just something that helps the melting of a glaze. In most situations this definition works out well, but you have to remember that a small amount of alumina helps silica melt, or that a bunch of silica can cause the alumina to melt.

A good general definition of "flux" is "something that helps melting".

The water melts the alcohol just as the alcohol melts the water.

If your system has more water than the eutectic mixture, the mixture with the lowest melting point, then as you cool further and further the excess water will crystalize first as ice. If we were talking about a glaze we might say that the mixture devitrifies and become a matt or crystalline "glaze". As you cool this mixture the water will crystalize out first. When you cool below that eutectic point it all freezes. Sometimes the cooling of such a mixture will result in a solid solution of both materials rather than a mixture of different crystals.(check).

In order to understand the dynamics of ash, you have to understand the dynamics of the materials in the ash. Functionally the most useful division is described in the Periodic Table of Elements. The alkali metals, sodium and potassium sit in the first column of the Periodic Table. Compounds of both of these are highly soluble in water. They have melting points just above 1500 degrees farenheit. Rain water can leach them from rocks and from wood. When heated above their melting points they are given off as fume. This starts to happen at their melting temperature and happens more rapidly as the temperature increases. They are generally similar in action in most glazes.

An important part of all of this is that both of the oxides, Na2O? and K2O? will form a liquid with alumina and silica below 1500 degrees F. The glass that results is not durable and often devitrifies, but it is a glass. With American Shino glazes that include soluble soda or potash, these materials deposit on the surface of the glaze coat and form a glass at a temperature low enough to trap the carbon dioxide given off by calcium carbonate. It is this that I believe causes the "Shino first or fear the worst" sort of blisters.

People fueling kilns burn many sorts of materials. The fossil fuels are used by many. Ground coal can be used in some circumstances. With it comes a host of materials. The ash is quite high is silica. When you think about how it was formed, at the bottom of a swamp, that most of the potash and soda are washed out makes a lot of sense.

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Page last modified on August 03, 2022, at 08:24 AM