Most people who want to prepare wood or plant ashes as a
glaze ingredient will certainly know as much about it as I do.
Probably more. But it is a thing I am sometimes asked about,
so these few notes are addressed to the tyro. I do not imagine they
will be of any use to the old hand, but they might save some
trouble to anybody new to the game.
If you want the ash from your burning to be pure, burn it on a clean floor, preferably concrete or stone. If you burn it on the garden or on gravel some colourant-probably iron oxide- will get into the ash; the result may be pleasant, but it will not be typical of the ash of that particular plant.
Burn the material till it is reduced to a fine ash, stirring the pile at intervals to let the air in. If it is wood you are burning the ash will probably be pale grey, pale fawn or white; if it is any sort of straw, grass or herbacious plant it will usually be dark grey or black. When the ash is cool shake it dry through an ordinary wire cooking sieve: if you do not do this fragments of half burnt material left in the ash may damage the fine lawn in the next sieving. Mix the ash with water and resieve it through 120 mesh lawn, using plenty of water and not trying to push through the lawn anything that is unwilling to go. Ash forced through a sieve always floats up to the top of the water and is poured off in wash- ing; so you lose your trouble and probably spoil your sieve. Leave the ash to settle (it usually takes about four hours) and then pour or siphon off the water, which may be brown or covered with a grey scum. Add fresh water and go on pouring off until the water is perfectly clear (four or five washings).
Dry off the ash. An old "biscuit" bowl is useful for this, as the porous biscuit helps to get the moisture out. When it is perfectly dry powder it is ready for use.
If you want to use bonfire ashes-e.g. from a pruned hedge or a burning of undergrowth in a wood-it is important to get the ash before it has been rained upon, and to pick up only the top layers if you want the ash to be reasonably pure. The lower layers will have collected all sorts of colouring matter from the ground. They can be good fun but they are not pure ash. It is very well worth while to look out for bonfires if you want to make ash glazes in any quantity; because one very soon finds out that one needs a considerable pile of rough ash to make quite a small quantity of fine ash ready for use.
To produce the typical colour and texture of any specific ash glaze it is, I think, usually necessary to fire it at a fairly high temperature - 12500C, or over-preferably in a reduced fire; and to use the ash in a fairly high percentage, probably 25-40%. From my own experience, admittedly very limited indeed, I would say that ashes develop their special characteristics most fully, and most beautifully, in a kiln where the pots are packed in saggars and where the fuel used is wood. There seems to be a special corres- pondence between the wood ash in the glaze and the firewood ash particles flying through the kiln. But other potters' experience may be quite different.
The chemical composition of ashes varies very widely: there may be anything between 2 and 50% of silica in an ash, between 3 and 60% of lime. There is usually between 15 and 30% of potash; and the other ingredients are normally magnesia, phos- phoric acid and iron in small quantities, with manganese dioxide, soda and chlorine in smaller. However, as there are likely to be trace elements as well, and as all the ingredients probably vary in proportion with the soil in which the plant is grown and the time of year at which it is cut, it is obvously better, and much cheaper, to eschew analysis and put plenty of tests in the kiln. Most potters have their favourite glaze formulae; and to anyone who wants to play with ashes for the first time I would suggest that he should substitute the ash for the limestone or whiting in his usual glaze: he will probably find that it lowers the temperature slightly if he normally uses limestone, and raises it slightly if he uses whiting. But this is a very tentative suggestion: it is merely the sort of thing I do myself.
Finally I would suggest to my hypothetic tyro that he should not expect too much: quite a lot of tree and plant ashes make glazes very much alike, and quite a lot of them are rather dull. Even the occasional beauties are more insinuating than startling. It is also true that, as ash glazes are in most formulae semi- transparent, the colour of the body beneath affects the colour of the glaze. So that two potters using the same formula and the same ash may easily get quite different results if they use different bodies, just as they may if they use different kilns. Pottery, as practised by the studio potter, continues to be a rather chancy affair.