Bruce Girrell on mon 10 sep 01
Well, after raising my hand in class so quickly and almost, but not quite,
getting the answer right, the teacher made me go home and check out my
references. I found a nice description of the origin of the cone numbering
sequence in _The Potter's Dictionary of Materials and Techniques_ by Hamer
and Hamer.
To avoid misrepresenting the material, I have copied it verbatim below.
Dr. Seger took his formula for feldspar as a starting point. This was
K2O-Al2O3-6SiO2. He altered the alkaline oxide to include calcium oxide and
by reducing the potassium oxide content he was able to alter the aluminum
oxide:silica ratio to 1:10. It is worth noting that this ratio is a common
one in glazes. The aluminum oxide equivalent was now 0.6 molecules and the
silica 6 molecules. This cone was therefore given the number 6 and
corresponded to the slow maturing point of feldspar at 1200 oC (2192 oF) or
a quicker firing to 1250 oC (2282 oF).
By adding 0.1 molecule of aluminum oxide and one molecule of silica at each
step the progression of cones was numbered upwards in ones. Each step was
subsequently found to relate approximately to 25 oC (45 oF). Dr. Seger next
worked downwards progressively to number 1 with additions of iron oxide and,
later still, alterations were made to the alkaline oxides. To denote the
change of alkaline oxide, the cones were given the suffix A with the number.
Later still, someone else extended the range below 1A and the numbers given
were 01A, 02A etc. These numbers therefore increase down the temperature
scale whereas Seger's numbers increased up the scale. Also they do not refer
to any molecular equivalents within the recipes but are merely numbers.
For those of you who don't have a copy of _The Potter's Dictionary_, the
above represents the kind of gems that you find throughout the entire book.
The book is not only useful in a technical sense, but also very interesting
to read. Because it is organized in alphabetical order, there is no plot.
You can pick it up and select any topic at random. Almost certainly you will
learn something new that day.
Bruce "that's better" Girrell
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