Rod, Marian, and Holly Morris on thu 7 oct 99
------------------
I teach beginners with terra cotta, and my commercially mixed clay often
=22chalks=22 in the fire. I presume this is because of the lack of barium in=
=22school
safe=22 clays. There was a reference awhile back in CM about using barium =
sulfate
(the inert stuff you drink prior to x-rays) rather than the big nasty =
(barium
carb) to bind up the sulfates in the clay . What would be the downsides of =
me
mixing some barium sulfate into my slurry bucket so that at least my =
recycled
clay would not be chalky? Any other ideas? Getting a new clay source isn't a
great option for me since I use relatively small quantities, and shipping is
outrageous.
Greetings from Marian, freezing in Northern Michigan
Gavin Stairs on fri 8 oct 99
At 20:19 1999.10.07 , Marian Morris wrote:
>----------------------------Original message----------------------------
>------------------
>I teach beginners with terra cotta, and my commercially mixed clay often
>"chalks" in the fire. I presume this is because of the lack of barium in
>"school
>safe" clays. There was a reference awhile back in CM about using barium
>sulfate
>(the inert stuff you drink prior to x-rays) rather than the big nasty (barium
>carb) to bind up the sulfates in the clay . What would be the downsides of me
>mixing some barium sulfate into my slurry bucket so that at least my recycled
>clay would not be chalky? Any other ideas? Getting a new clay source isn't a
>great option for me since I use relatively small quantities, and shipping is
>outrageous.
Hi Marion,
Won't work. The reason is easy to explain: Barium carbonate dissolves
(some). Barium sulphate doesn't (mostly). What happens is that easily
dissolved calcium sulphate exchanges with the barium, in solution,
resulting in calcium carbonate and barium sulphate, neither of which is
very soluble. So they stay put when the piece dries. The calcium sulphate
scum is a result of migration of moisture with dissolved salt to the
surface to evaporate, leaving the salt behind as a surface deposit. Two
solubles combine to form two insolubles, and solve the problem. Starting
with one of the results won't get you anywhere.
This is a rather beautiful precipitation reaction. Precipitation, or the
formation of a solid from a solution, is often used to separate out a
particular ion. The beauty of this reaction is that both reagents come out
together. Not much use for analysis, but great for getting rid of scum, or
calcium suphate efflorescence.
Gavin
Mark & Pauline Donaldson-Drzazga on fri 8 oct 99
----- Original Message -----
From: Rod, Marian, and Holly Morris <>
To:
Sent: 08 October 1999 01:19
Subject: Un-chalking Terra Cotta
----------------------------Original message----------------------------
------------------
I teach beginners with terra cotta, and my commercially mixed clay often
"chalks" in the fire. I presume this is because of the lack of barium in
"school
safe" clays. There was a reference awhile back in CM about using barium
sulfate
(the inert stuff you drink prior to x-rays) rather than the big nasty
(barium
carb) to bind up the sulfates in the clay . What would be the downsides of
me
mixing some barium sulfate into my slurry bucket so that at least my
recycled
clay would not be chalky? Any other ideas? Getting a new clay source isn't a
great option for me since I use relatively small quantities, and shipping is
outrageous.
Greetings from Marian, freezing in Northern Michigan
Dear Fellow Clayarters,
the simplest solution is to fire higher and get rid of the soluble salts in
the clay. This also improves the strength. I fire my terracottas to 1170 C,
totally frost proof and durable.
Happy potting Marek http://www.mpley.uk.com
Ron Roy on sat 9 oct 99
Barium sulphate will not work to lock up the salts in clay - too stable -
only Barium Carb will work I have been told.
RR
>----------------------------Original message----------------------------
>------------------
>I teach beginners with terra cotta, and my commercially mixed clay often
>"chalks" in the fire. I presume this is because of the lack of barium in
>"school
>safe" clays. There was a reference awhile back in CM about using barium sulfate
>(the inert stuff you drink prior to x-rays) rather than the big nasty (barium
>carb) to bind up the sulfates in the clay . What would be the downsides of me
>mixing some barium sulfate into my slurry bucket so that at least my recycled
>clay would not be chalky? Any other ideas? Getting a new clay source isn't a
>great option for me since I use relatively small quantities, and shipping is
>outrageous.
>
>Greetings from Marian, freezing in Northern Michigan
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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