ash glaze chemistry ?

Beowulf wrote in news:pan.2004.02.26.12.50.49.43834 @nowhere.net:

Quote from Glazes for the Studio Potter, Cooper and Royle

'The use of wood ash and plant ash as a glaze ingrediant dates far back in antiquity and it was the Chinese who discovered that at a higher temperature the ash from the wood used to fire the kiln settled on the surface of clays to form fairly simple, but often attractively mottled glazes. (For modern examples have a look for pottery that has been fired in an anagama kiln)

Ashes contain, in a fine state of division, large proportions of silica, some potassium and sodium salts, alkaline earths, iron and often phosphate salts. The amounts and varieties of these minerals vary considerably from plant to plant, soil to soil, with the same plant, season to season.

When combined with feldspar or clay, wood or plant ash will form richly textured and coloured glazes from about 1200c upwards.

Two recipes are ash 2, feldspar 2, china clay 1 ash 2, feldspar 2, ball clay 1'

So when you see a pile of ash that you want to use in a glaze what you need to realise and look at it as a group of minerals and nothing else. These minerals will react in exactly the same way as the same mineral from another source eg silica. So the chemistry in ash glazes is no different than the chemistry using any other glaze materials.The only problem is of course is that the chemical composition of each batch of raw ash will differ from the last. The romantic appeal of using ash makes it a favourite material for the stoneware potter but it is absolutely hopeless for the potter who requires consistent glaze results.

Another organic material used in glazes and in clay bodies is bone ash

I would suggest that you go to

If you would like to discuss glaze chemistry further email me at snipped-for-privacy@slingshot.co.nz

Regards

John Webb

On Thu, 26 Feb 2004 20:34:05 +0000, Uncle John wrote:...

..

Thank you so much John. Good info. As a computer geek I will return the favor by suggesting a way to hide your email address (not from me, I really appreciate your sharing it) from spam bots that roam newsgroups like these to collect emails to add to spam lists. A good way to give out an email and yet hide it from spam bots is perhaps johnwebbATslingshotDOTcoDOTnz Then again maybe you have a really goot spam filter. Me, I am sick of all the spam clogging my mailboxes, though I do a good spam filter that routes it into a spam folder.

~Beowulf (Randall)

Just to add an interesting tidbit to John's excellent discussion, it is apparently the case that the ash from slow-growing plants (hardwood trees) contains less silica than that from fast-growing plants. This seemed counter-intuitive to me when I first read it, since I think of silica as strong and hard like hardwoods. I don't have a definitive botanical answer for this, but in thinking about it perhaps the silica is a "cheap trick" for fast-growing plants (like annual weeds) to get adequate strength even from tissue that isn't very dense, whereas the hardwoods get their strength from wood density. Alas, my botany expert passed away a couple of years before this question arose. Anyone know the answer?

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Bob, I do know that when I was at college back in the middle ages my Ceramics Tutor The late (and great) Paul Barron classified wood ash in 2 classes Soft or Hardwood Ash and Hard or Softwood Ash. The contra sounding terms always tickled my sense of humour, and I believe you are right in that it reflects the Silica content.

Steve Bath UK

In article , Bob Masta writes

Steve Mills wrote in news: snipped-for-privacy@mudslinger.demon.co.uk:

To quote the previous reference

Ashes can be classified by several means. The useful classification is hard, soft and medium ahes. Generally speaking, the quicker the growth, the higher the proportion of silica and the harder the ash. The longer the growth, the softer the ash;as a result the hard woods, which have taken many years to grow, yield low silica and high flux content, such as potassium and calcium, and gived soft ash. Quickly grown plants such as bracken, give ash with a high silica content and a hard ash. However, the silica content of ash is often intimately combined with other ingrediants and does not act as a refractory material in the way flint or quartz acts.

Classical combinations of materials in glazes include feldspar, clay and flint. Small quanitities of calcium carbonate (whiting) or dolomite are often added. Ash is usually considered to be a fluxing material.

End quote

Regards

John Webb