iandol on tue 26 jun 01
A recent posting asked about the potential for corrosion by kiln =
effluent when using Sodium Carbonate.
Now, I am uncertain which of the various notions are promoted as the =
chemistry of this process by those teaching or demonstrating Soda =
Glazing so what I say is conjectural, but it seems as though an analogy =
to that used to explain Salt Glazing is being promoted. In the =
HYPOTHESIS put forward by Daniel Rhodes, he assumed that Sodium Chloride =
reacted with water to give Sodium Monoxide and Hydrogen Chloride. Hence =
by analogy, those who promote using Sodium carbonate consider a reaction =
with water to give Carbonic Acid and Sodium Monoxide. There is also a =
HYPOTHESIS which promotes Decomposition by heat insisting that Sodium =
carbonate decomposes to Sodium Monoxide and Carbon dioxide when it is =
thrown into the kiln. The arguament for this hypothesis is that sodium =
carbonate must behave like Calcium carbonate and everyone knows whiting =
decomposes when it is heated.
My chemistry text (General and Inorganic Chemistry, P.J. Durrant. P415) =
tells me that Sodium Carbonate is fused with Silica to make Sodium =
Silicate (used in a solution with water by potters to deflocculate =
clay!!) which is water soluble (Solution recommended as an adhesive =
cement for "China"). P. J. Durrant also reveals that when heated above a =
red heat in Platinum crucible (P319, footnote), weight loss is extremely =
slow but when heated in a porcelain crucible weight loss is rapid =
because porcelain contains silica which expels the carbon dioxide from =
Sodium carbonate.
Fear about Corrosion seems to arise from the idea that any unused Sodium =
monoxide will react with water vapour to form Sodium hydroxide, which is =
exceptionally corrosive.
It seem possible that Corrosion can come from two processes. The first =
is direct solution of a metal by an acid, as with Zinc and Hydrochloric =
acid or by an alkali, as in Aluminium (bright metal) and Sodium =
hydroxide. Both reactions yield Hydrogen. Alternatively, there is =
another route, post firing, which requires three things. One is a =
chemical which ionises when it dissolves in water. The second is a =
source of moisture. The third is a metal which has sufficient impurities =
to create microscopic electrolytic cells. Any metal will do but it is =
possible that both Zinc and Aluminium form self protecting oxide =
surfaces. Check this as a fact. Exposed Iron or steel are ideal =
candidates for corrosion by Sodium chloride and Sodium Carbonate. Both =
compounds ionise in water and therefore solutions can carry electric =
current. It is a known fact that both of these chemicals are expelled =
from a kiln stack as particulate matter so there is always potential for =
corrosion to occur when they settle on a metal surface which will get =
wet. Sodium chloride solution can invade the protective oxide coating =
usually found on aluminium which would encourage future corrosion.
I hope those that need information find this useful.
Best regards,
Ivor Lewis, Redhill, South Australia
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