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clay which will not take salt.

updated wed 27 sep 00


iandol on mon 25 sep 00

Dear Vivek,
You seem to be having a lot of difficulty with sewer pipe production.
If you have just changed your clay supplier or your regular supplier has =
changed the product he is supplying, then things may happen which are =
unanticipated. Ask about this clay. Ask about the free silica content =
and the total silica content. Ask about the flux content, in particular =
it's potassium and iron oxide content. Ask about the distribution of =
clay to none clay minerals. Ask about the particle distribution. Ask =
about any other chemicals in its constitution.
One problem is the term Fireclay. Fireclay is not a substance of uniform =
or standardised composition. It is called fireclay because it has a =
higher heat resistance than many other clays such as terra cotta. =
Sometimes it is not very plastic but may be coarse and granular.
Heat resistance can be achieved in three ways. This clay may have a high =
alumina content, or this clay may have a high silica content, or this =
clay may have a very low Flux content. The composition of the clay will =
depend upon it's history. Many fireclay types are found underneath coal =
seams. They are the remnants of the soils in which the carboniferous =
plant life grew. The plants used the soluble alkali and alkali earth =
elements and extracted them from the soil. All of these conditions =
contribute to making a substance which will not accept salt or soda and =
form so form a glaze surface. Further more, all of these clays need to =
be heated to a substantially higher temperature to make them fully =
Clays which are high in Potassium compounds such as muscovite mica and =
potash felspar give better salt surfaces than those which are deficient =
in this elements, they also vitrify at lower temperatures than highly =
refractory fireclay.
Calcium carbonate is insoluble in water. Calcium sulphate is sparingly =
soluble in cold water.
Magnesium carbonate is almost insoluble in water. All of these =
carbonates are soluble in acid solutions. Excess manganese compounds =
would colour the clay and would act as fluxes, so it is unlikely that =
these are problems. Is your water acidic, sometimes called "soft water"
I would be interested to know why you have chosen to use a fireclay for =
pipe production. When I had dealings with the sewer pipe makers in the =
North of England and Scotland, they used seat earths form the coal =
measures which were called fireclay. But they were called fireclay =
because of their high silica content. Most seemed to contain high iron =
so they colour ranged from dark tan to black. From time to time the clay =
would change as some seams were abandoned and new ones revealed. I =
recall at Eltringham that there were occasions when the clay became very =

I am concerned that the notion of high measures of free silica being =
necessary for the production of a good salt glaze is perpetuated without =
the support of adequate research and chemical analysis. Simple tests in =
any salt kiln will reveal that salt has no effect on raw alumina, raw =
silica or kaolin which has very low alkali residuals. Salt will give a =
good glass on the ball clay I have tested, with orange colouration and =
surface texture. This must be due to the residual mica and other =
potassium bearing compounds. Clay recipes in books about this topic and =
other glaze recipe books always seem to include Potash felspar among =
their ingredients. It is also known that potassium chloride contributes =
significantly, up to fifty percent of solids, in salt kiln effluent.

Without more information I can give you no adequate answer.
My best regards,
Ivor Lewis