Yesterday I stumbled into the last hours of a show where I had some wood-fired ceramic plates hanging. Each of the artists there mused a little on the topic of wood-firing. Tony Moore, who organized the exhibition, asked me to speak a little about the firing process, and to be specific on a few technical points.
I noticed this big sense of mystery from people about wood ash. Was it actually glazing pots, and giving them their colors? In a word, yes!
In a wood fired kiln, clay pots are 'fired' by heat liberated from burning wood. The wood burns as it would in an ordinary fireplace, but the heat accumulates, and builds up, due to the efficient insulating design of the kiln. Near the end of a typical wood-firing, kiln temperatures are over 2200 degrees, or hotter. Logs chucked into the kiln through the stoking ports catch fire the instant they cross the threshold. I always come home from my firing shift with a sunburn.
Eye protection is a must, unless like me you believe in your body's ability to look into an energy source and 'keep' the energy. That is a process I learned from looking into the sun regularly, as a form of meditation. Why not a kiln? So at our last firing I checked all the cones with bare eyes. I don't recommend this. . . but it can be learned. The meditation is one of borrowing energy from the kiln, and converting it, into psychic energy at the back of the eyes. I've never been injured by any of this. In fact I feel healed to a degree, as I always notice that I don't need glasses for a day or two afterwards.
Pots can't burn. They are made of oxides - all the chemical energy of their minerals has been released already in combination with oxygen, and carbon. But pots can melt, and do. I've seen firings where not only pots melted, but kiln shelves, which are a much higher temperature ceramic, and even the interior of a kiln chamber itself.
This only happens on the rarest occasions. The amount of energy required to lift a ton of material one hundred degrees, from say boiling temperature to 312 degrees, is only a tiny fraction of what is needed to go from say 2200 degrees F to 2300 F. Kiln meltdowns are almost always due to huge 'pilot error', or a complete breakdown in safety procedures. You have to work hard to melt down a wood kiln.
I mention this to illustrate a concept, which is how well different materials hold up to heat.
That measure - of how well a material holds to high heat - is termed by potters, 'refractory'. How refectory is this clay or that clay? All clays are refractory, wood isn't. Wood is a fuel, and just needs the kiss of ignition to release energy, whereas clay has no internal chemical energy, at least not in earthly reactions with oxygen. It's inert stuff, which, with heat, may be turned back into rock.
Potters are geologists, reforming stones, making dishes and sculptures and all manner of objects out of the same materials as the earth's crust.
So up comes the common question: 'How does wood-ash alter the color or firing of a clay pot?'
We're conditioned to think that 'wood ash' is some leftover piece of weakness. After all, if the stuff didn't survive a fire, how can it play again into the fire equation, making pots all over again? Well the point is that wood ash did survive the fire. Ash is made of incombustible minerals, oxides mostly, of the compounds found in wood. All the cellulose has burned off, producing the two major by-products of any flame, carbon dioxide and water. Plants, trees, all life forms, are composed of many other minerals as well. Many of these do not exit the kiln as a gas, but rather move through it as specks of solid ash. Even these minerals that do take on a gaseous form, also contribute to the 'melt' of the pots. Silica and alumina, are the two minerals found in most clay. Silica is a glass former, alumina rigidifies, and withstands high temperatures. Potassium, phosphorus, calcium and magnesium carbonates or oxides, as well as oxides of iron, copper, and other metals, are are all present in wood ash. Each of these may function as a flux, that is aid in the melting of clay, or in glass forming.
Wood ash at any temperature is a highly reactive alkaloid, and may be mixed with water to produce lye for soap, or by gardeners to increase potassium levels, and restore the pH balance of acid soils.
The most basic formula for clay is composed of two major components, silica and alumina, though most clays, even the whitest porcelain, harbor a host of other impurities in small concentrations. So is the same for most of our planet. The rock of the earth's crust, the globe we live upon, has almost the exact chemical formula and balance of minerals as a fired piece of ceramic ware, with a glassy film at the surface, colored like a crusty glaze, and some slippery stuff clinging to it, called life.
Wood ash is particularly rich in silica as well as other minerals that all behave as 'fluxes', that is they will further lower the melting points of other compounds which they contact, including silica itself, and will weaken alumina's ability to 'refract' or stand up to high heat.
That's what bits of wood ash do to a pot. They fly through the yellow hot kiln, sticking to pots, and begin fluxing, or melting the surface of clay or glaze. The silica and other minerals in wood ash has fluxed the silica that's in the clay, and 'vitrified' it, i.e. made it glassy. Here's an example of extreme fluxing of wood ash and clay at the surface of an unglazed plate. Green glass forms.
Now where did the silica in the wood ash come from?
Let me rephrase the question. Why do plants contain silica?
Ahh . . . this is the interesting part . . . .
Plants, trees and grasses employ silica as a natural defense system, and use it in the same way that broken glass is stuck into the tops of concrete walls to keep trespassers out.
Have you ever been cut by a blade of grass?
Under a microscope, the leaves of Big Bluestem, or Turkey Grass, (the predominant grass growing in the tall-grass prairie of the American west), show a jagged edge, tipped with minuscule shards of silica, called phytoliths, or 'plant stones', arranged like shark's teeth, that will cut more cleanly through flesh than any knife. Why? To keep the number of grazing animals at bay. The toughest grasses, contain the most silica. Big Bluestem, was eaten by bison only, and even then its silica content was responsible for eventually wearing down the bison's massive molar teeth, making it impossible for them to digest their food, and eventually leading to their individual deaths.
Here the food source was the agent responsible for regulating the population of the creature that predated upon it. The grass kept the buffalo population in check. It needed the buffalo, to seed, fertilize, plow, move other beneficial plant species in and out and around it's tall leafy ecosystem, but as a species it purposefully integrated a hostile mineral to shorten the average life of its predator.
Silica makes plants tough, difficult to chew and digest. An inorganic molecule from the earth has evolved into part of the plant's defenses. Plants are ceramicists too.
Do plants build kilns or insulating blankets from silica? Oh yes!
Tree bark protects a tree from fires, as much as from browsing deer that might take a tasty chomp. The foamy thick bark on trees like pine and oak, will protect an old tree from the fiercest forest fires. In fact many species of trees will only drop seeds after a forest fire, when soil conditions are right. Remember the wood ash raises the pH of the soil, back towards neutral. Conifer forest soils have a tendency to become very acid. After a fire the pH goes up again, ideal for young shoots!
In order to protect its own life the silica has been imported by largest trees from the ground, at a high energy cost, and arranged by the bark into a foamy composition, not unlike soft fire brick or soft ceramic blanket, just for this purpose.
Bark has much more of the fluxy stuff that ceramicists look for than plain hardwood, which tends to burn clean.
Rabbits, deer, and birds nibble new shoots where the silica rich bark hasn't yet developed. Beavers, who chew bark and have teeth that grow throughout their lives, save on dental bills by preferring young poplars and birches.
What of other plants? Do they all use silica in the same way?
No. Some pay off predators by providing edible fruit in huge quantities. Why bother eating leaves and stalks if berries are plentiful?
Some make their leaves and sap extremely bitter. Again these bitter compounds are alkaloids, which also makes them poisonous, or healing, or both, depending on dose.
Some taste terrible, foul even . . . some reward with a psychoactive compound.
Others grow thorns . . .
