Tom Buck on thu 24 jun 99
The question was asked: Can strontium carbonate be used in
place of barium carbonate to halt the migration of soluble
salts to the surface of a drying pot and later show as an
unpleasant, glaze-changing "scum" on bisqued ware.
Ron Roy answered: No, strontium cabronate will
not do the job that barium carbonate (1% addition, or
less) will do in a prepared (wet) claybody containing
soluble salts.
For those that want some nitty-gritty, the data
will be found in that sub-discipline of chemistry
called Solution Chemistry. In brief, Barium Carbonate
is slightly soluble, enough to release some Ba++ ions
the water in the claybody being shaped. In most
cases the soluble salt(s) involved are sulfates of
calcium, magnesium, sodium and potassium, although in
some "hard" water (eg, well water) there also will be
present some nitrates/nitrites and carbonates of the
same alkaline earth and alkali metals.
Barium carbonate will halt scumming because
it is soluble enough, 20-40 milligrams per Litre (or
parts per million), to provide Ba++ ions to link
with SO4-- ions in the water, and once so linked
these form the molecule BaSO4 (barium sulfate), and
this new molecular form is 10-fold less soluble,
2-3 mg/L, so the sulfate stays internal and almost
no soluble salts migrate to the surface.
But the same is not true for strontium
carbonate. In cold water (15 oC) SrCO3 has a solubility
of 11 mg/L; in hot water (50-60 oC), the solubility jumps
to 650 mg/L. If some Sr++ ions linked with SO4-- ions,
not many would precipitate out as SrSO4 since its
solubility is 113 mg/L (cold) and 140 mg/L (hot) which
means the Sr++ and SO4-- ions would migrate to the
surface along with Ca++, Mg++, and SO4-- ions. Hence,
the scum will be there on biscuit, only now some
strontium material (SrO) will join the CaO and MgO to
form the scum.
If a potter wishes to use a natural, local
clay and it is likely to produce scum on biscuit, then
the remedy may be to "wash" the clay with high quality
water (almost mineral-free). First dry the dug clay, then
make a thin slip using the good water, and stir the slip
for many minutes to allow the water to remove adsorbed
ions from the clay particles. Then collect the washed clay
particles on filter cloth/paper, and prepare the claybody
from the wet slop.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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