ash glaze chemistry ?

Can anybody tell me what chemistry is involved in ash glazes? Chemical
reactions involved, atoms or molecules involved, etc.? I am a biologist
devloping a "Biology Through Art" course and would like to incorporate
ceramics in some way, thinking perhaps ash glaze since it involves wood
(plant biology), seeking any info on what substances in the ash itself is
involved in the glaze or glaze reaction. Thank you.

Beowulf wrote in newsan.2004.02.26.12.50.49.43834
@nowhere.net:

Can anybody tell me what chemistry is involved in ash glazes? Chemical
reactions involved, atoms or molecules involved, etc.? I am a biologist
devloping a "Biology Through Art" course and would like to incorporate
ceramics in some way, thinking perhaps ash glaze since it involves wood
(plant biology), seeking any info on what substances in the ash itself is
involved in the glaze or glaze reaction. Thank you.


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 www.digitalfire.com and read through their
articles on glaze chemistry.

If you would like to discuss glaze chemistry further email me at


Regards

John Webb

On Thu, 26 Feb 2004 20:34:05 +0000, Uncle John wrote:...
If you would like to discuss glaze chemistry further email me at

...

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)

On Thu, 26 Feb 2004 06:50:49 -0600, Beowulf
wrote:

Can anybody tell me what chemistry is involved in ash glazes? Chemical
reactions involved, atoms or molecules involved, etc.? I am a biologist
devloping a "Biology Through Art" course and would like to incorporate
ceramics in some way, thinking perhaps ash glaze since it involves wood
(plant biology), seeking any info on what substances in the ash itself is
involved in the glaze or glaze reaction. Thank you.


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
www.daqarta.com

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
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
www.daqarta.com

--
Steve Mills
Bath
UK

Steve Mills wrote in
:

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

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

http://www.digitalfire.com/
is a site you might want to check out...


"Beowulf" wrote in message
news
Can anybody tell me what chemistry is involved in ash glazes? Chemical
reactions involved, atoms or molecules involved, etc.? I am a biologist
devloping a "Biology Through Art" course and would like to incorporate
ceramics in some way, thinking perhaps ash glaze since it involves wood
(plant biology), seeking any info on what substances in the ash itself is
involved in the glaze or glaze reaction. Thank you.

www.digitalfire.com is very useful for these kinds of questions

http://www.digitalfire.ab.ca/cermat/material/2138.html

Is the page for wood ash

DKat

"Uncle John" wrote in message
...
Steve Mills wrote in
:

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

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