Charles G Hughes on wed 22 mar 00
------------------
With these three responses I am both encouraged and worried. How much do I =
worry
about the dangers of causticity? I have a bunch of this Calcium Oxide and =
would
really like to be able to use it. However safety first, it would be in a =
public
studio. I can't have students frying their fingers.
Michael banks said
=22Calcium oxide (quicklime or calcia) is a strongly basic (alkaline) oxide =
and
reacts with water releasing a lot of heat. As it is so caustic, it should
not be handled without hand and eye protection. Calcium oxide is not the
same as wollastonite (calcium silicate) and is probably too soluble and
caustic to use in glaze. Whiting (calcium carbonate) is de-carbonated above
825oC to calcium oxide anyway and should be concentrated enough to provide
any reasonable amount of lime to a glaze, as well as being a lot more
friendly to the user and insoluble too.=22
Tom Buck said
=22 Use the lab-grade CaO (lime, caclium oxide) in place of whiting,
calcium cabronate, CaCO3. CaO mole weighs 56=3B CaCO3 mole weighs 100, so
adjust the weights as needed in the recipe. Wollastonite is Calcium
Silicate, CaSiO4=22
Kat Berger said
=22Remember that wollastonite is calcium and silica. Off hand, I believe
it's equal parts of each but don't quote me on that. They say that you
can substitute calcium and silica for wollastonite but I actually have
tried that and my glazes would come out slightly different. You know,
when the glaze just isn't working exactly the same. So, I assume that
in very touchy glazes it becomes difficult to substitute but in more
stable glazes you can.
.
Michael Banks on thu 23 mar 00
Charles, I think your biggest problem with using quicklime (CaO) as a raw
glaze ingredient is going to be the solubility. The alkaline pH of this
material is a factor too, but I'm not going to belabour the point. (But
just a macabre note here: quicklime is used to get rid of human bodies in
mass-graves in wars, genocides and natural disasters). (But on the other
hand, some live people in Asia also put a little quicklime in their
mouths.... -to neutralise the drug betel nut, so it can't be quite as bad as
caustic soda)!
When you add CaO to an aqueous glaze it reacts with the water exothermically
to produce calcium hydroxide solution. The vigour of this reaction is
determined by the grade of quicklime, but as you have lab material, it is
likely to be analytical grade -the highest. This resulting limewater (CaOH)
solution will soak into any porous bisc ceramic body during glazing and the
calcium content will act as a body flux. This situation is identical to
using soda ash, borax, or any other soluble flux material. Such soluble
glazes are usually only used freshly mixed (because the salts undergo ionic
exchange with each other and tend to precipitate coarse granular crystals,
as well as often setting like concrete. With flux-soluble glazes, provision
has to be made to use a body that can stand up to the additional internal
flux.
In conclusion, I would say that when you that when you have access to a lot
of a cheap and common raw material widely used in industry (like
quicklime) -that is rarely used in ceramics, there is generally a good
reason for this...
Michael Banks,
Nelson,
New Zealand
From: Charles G Hughes wrote:
----------------------------Original message----------------------------
With these three responses I am both encouraged and worried. How much do I
worry
about the dangers of causticity? I have a bunch of this Calcium Oxide and
would
really like to be able to use it. However safety first, it would be in a
public
studio. I can't have students frying their fingers.
Michael banks said
"Calcium oxide (quicklime or calcia) is a strongly basic (alkaline) oxide
and
reacts with water releasing a lot of heat. As it is so caustic, it should
not be handled without hand and eye protection. Calcium oxide is not the
same as wollastonite (calcium silicate) and is probably too soluble and
caustic to use in glaze. Whiting (calcium carbonate) is de-carbonated above
825oC to calcium oxide anyway and should be concentrated enough to provide
any reasonable amount of lime to a glaze, as well as being a lot more
friendly to the user and insoluble too."
Tom Buck said
" Use the lab-grade CaO (lime, caclium oxide) in place of whiting,
calcium cabronate, CaCO3. CaO mole weighs 56; CaCO3 mole weighs 100, so
adjust the weights as needed in the recipe. Wollastonite is Calcium
Silicate, CaSiO4"
Kat Berger said
"Remember that wollastonite is calcium and silica. Off hand, I believe
it's equal parts of each but don't quote me on that. They say that you
can substitute calcium and silica for wollastonite but I actually have
tried that and my glazes would come out slightly different. You know,
when the glaze just isn't working exactly the same. So, I assume that
in very touchy glazes it becomes difficult to substitute but in more
stable glazes you can.
.
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