Ivor and Olive Lewis on tue 19 dec 06
Dear Vince Pitelka,=20
You ask << Do you know if it is possible to have a high-alumina brick =
dense enough to prevent the sodium entry, but thermal-shock-resistant =
enough to stand up to the relatively quick heating and cooling in a =
studio kiln?>>
I'm not sure about that but I find it interesting that your Kruzite =
bricks are in the region of 70 to 80% Al2O3. This would put them in a =
composition range where the main constituent would be Mullite with a =
small fraction of free Aluminium Oxide (Corundum ?). Research done in =
England in the late 50's and repeated in the early 60's confirmed that =
effluent from industrial Salt Glaze Kilns contained a high proportion of =
Potassium Chloride. It also contained Aluminium chloride and Silicon =
Chloride as well as a few other unexpected and unanticipated elements as =
volatile chlorides.
This suggested to me that there must be direct chemical reactions =
between Sodium Chloride and the minerals in clay. (There is an =
interesting story that during WW1 when Potassium nitrate was in short =
supply to make explosives that the Charge fed into Iron Blast Furnaces =
should include Salt to displace Potassium as KCl which could be =
collected and used to make Potassium Nitrate). The thermodynamics of the =
potential equations and the volatility of the various salts support this =
idea.This may explain, in part, the degradation you observed.
There is a second possibility. We are all familiar with the notion that =
the Sodium from the Salt combines with Silica to make a vitreous =
substance but never stop to consider that there might be a reaction =
between Sodium chloride and Aluminium oxide or aluminous minerals. There =
is a chemical called Sodium Aluminate, NaAlO2 (which is the same as =
Na2O.Al2O3). Normally I would anticipate that this would be made by =
reacting an aqueous solution of Caustic Soda and Aluminium Hydrate or by =
dissolving Aluminium in Sodium hydroxide. Could Sodium Aluminate form =
when a kiln is being salted? If it did, it would have the potential to =
destroy a porous brick. Why? Well it is Hygroscopic and it would adsorb =
water from the atmosphere. Materials that adsorb water start to swell =
and exert pressure. This would account for your spalling. Sodium =
aluminate is also soluble in water so it could be leached out and bricks =
would disintegrate.
As far as I know Mullite refractories are not prone to thermal shock, =
unlike many fireclay refractories that contain free silica and undergo =
phase changes. Neither Mullite nor Corundum have phase changes. Also, =
high alumina refractories have about the same thermal conductivity as =
fireclay and that stuff puts up with intermittent firings.
I think you will appreciate that I have a very different concept of the =
Chemistry of Sodium Chloride Glazing which is why I put forward that =
notion of a Specification for a Protective Coating, a Kiln Wash, for the =
brickwork. Without that I fear a salt glaze kiln is predestined to self =
destruct regardless of the refractories that are used, with the possible =
exception of Zirconium Silicate !
I hope I have not bored you to tears with this technology but I do find =
scientific speculation enjoyable, even if I am wrong in the end.
Best regards,
Ivor
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