Tom Buck on thu 13 jul 00
Hi there June:
Since you ask for a bit more on what a RECIPE is, its total weight, its
preparation, etc. here is my take in an attempt to make this seem
sensible:
1) Essentially, when you put a glaze mix on the surface of a pot
and then heat it high enough to melt the glaze components, you are letting
ATOMS do their thing. Of course, most of the atoms are combined already
into the molecules that will particpate in the glaze making melting
process on the pot.
2) The key atoms in a glass are: Silicon, Aluminum (Aluminium),
Group 1 alkali metals (Lithium, Sodium, Potassium), and Group 2 alkaline
earth metals (Magnesiun, Calcium, Strontium, Barium), all of which will be
combined with Oxygen atoms to form glass. The oxide molecules come
together in very specific proportions (ratios) to make good glass (ie, a
good tough impervious glaze).
3) Since it is the oxide molecules that combine in these
proportions, our recipe (before the fact of melting) has to reflect the
molecular amounts, that is, the NUMBER of MOLES of each particpating oxide
(or molecular "species").
4) To mix a glaze, therefore, we have to use amounts of materials
that bear a proper relationship to the MOLES of each participant oxide in
the melt when it occurs. We need to weigh a set MASS UNIT that corresponds
to the Moles of Oxide in the glaze melt.
5) A complication is the fact that perhaps half our standard glaze
materials provide TWO or more Oxides that will melt into glass/glaze. So
we have to make allowances for that.
6) An atomic weight of Silicon is 28.1 mass units (use micrograms,
milligrams, grams, kilograms, ounces, pounds, hundredweight, tons, or any
other mass unit you choose). The scale in use is a real-world analogue of
the molecular world of the sub-microscopic (indivudal molecules). An
atomic weight of Oxygen is 16 and silica has two such atoms in its
molecule. Hence, the weight of SILICA (silicon oxide) is 28.1+16x2=60.1
which usually is rounded to 60.
7) Now a typical fine ground silica (flint/quartz) of 325 mesh (or
220 or 400 mesh) has a bulk density of approximately 1.2 grams per
millilitre. If you took a cup of this silica powder, ie, eight fluid
ounces, and packed the cup well (tamped the powder), you'd have just under
240 mL of material. So in real terms you'd be selecting 1.2x240=290 grams
of Flint/Quartz/Silica, and that amount is what would help form glass on
the pot. In effect, mixing a glaze by VOLUME is really obtaining the
weights of each component, providing you know within a reliable range the
density (ie, weight/volume) of the material (as above).
I think that's enough for now June. But if this prompts a followup
question, then by all means let's continue.
be well. Good Pots. Peace. Tom.
Tom Buck ) tel: 905-389-2339 (westend Lake Ontario,
province of Ontario, Canada). mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
GURUSHAKTI@AOL.COM on fri 14 jul 00
Hi Tom:
I already knew everything up to #7 . Loved the info on bulk density. Is there
a list somewhere giving bulk density on all these ceramic materials? It would
be fun to play with volume recipes at some point.
My question was asking whether what I was told years ago was true -- that
when you first calculate a formula and get that batch recipe, before you do
the division and multiplication to get it to 100%, is that batch formula one
that can be used both for 100% or volume. It never felt true to me--can't say
why, so I've never attempted to use volume, just stuck with the gram weight.
As I said in my response post to the original poster, I never could figure
out how to get 1.17 teaspoons of something!
So please, cherished and great, glaze guru, is it true or not?????
:-)
Warmest regards,
June
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