Tom Buck on mon 9 oct 00
Greg:
If you haven't already come to a decision on the 15 kilos of glaze
mix, then here is a way for you to check to see if you left out half the
silica/flint.
______________
Adjusting slips/glazes
Years ago, a technical specialist, M. Brongniart, was working at
the French pottery at Sevres that made (still does) the famous Limoges
porcelain foodware. Since slip-casting was the dominant method of forming
pots, he came up with a formula to determine/measure indirectly the amount
of solids in a clay slip so he could know how much water or clay to add.
It also works for glaze slurries. Cardew cites it as follows:
W = P-20 (g/g-1)
where W = dry weight of clay/glaze solids (raw materials);
P = the "pint" weight of the clay slip or glaze slurry in ounces;
20 is weight of pint of water;
and g is the density of the raw materials
Cardew goes onto explain (see Pg 269, Pioneer Pottery) that most
clay and glaze materials have a density of 2.5-2.6 grams per millilitre
(g/mL) and this assumption allows use of the formula with enough
accuracy for most purposes.
One can convert the formula to SI (metric) thusly:
W = [Wt - Ww] x (d/d-1)
where W is the weight of suspended solids (ie, raw materials) in the
weighed sample;
Wt is the total weight of a sample of the clay slip or glaze slurry;
Ww is the weight of water equivalent to the volume of the sample;
and d is the density of the raw materials.
The units of weight would be grams or kilograms; and the density units
would be any, as long as one uses the correct/appropriate value for the
density of water ...the usual density unit is grams/millilitre since
this leads to a density of water at 1 g/mL. (One could also use
kilograms/Litre and the value for water is still 1).
So one weighs say 200 mL of slip or slurry, determining the weight
to be 310 grams (+ or - 10). As an aside, this measurement yields a glaze
slurry a density of 310/200 or 1.55 g/mL, the typical value for dipping;
for spraying, a density is 1.4-1.45 g/mL).
Hence, the suspended solids content of the 200 mL sample is
W (in grams) = [310-200] x (2.5/2.5-1) = 110x1.7 = 187 rounded to 190
grams. By difference the weight of water is 120 grams.
Proving it to be ballpark:
190 g divided by 2.5 g/mL density = 76 mL (volume) of solids;
120 g divided by 1.0 g/mL density = 120mL (volume) of water;
and their sum is 196, rounding, 200 mL. This is the level of accuracy one
can get by assuming the raw materials all have a density of 2.5 g/mL.
In a factory situation where one works with tons of materials, one
would determine each individual solid's density value, and then using the
pecentages of the recipe one would compute the "real" density of the
combined solids (slip or glaze). And use this value in the Brongniart
Formula. The B Formula really does help when a studio potter is recycling
some clay slip gone too thick or a glaze slurry needing additions or
adjustments after it has been used/tested for awhile.
------------end------------
Now, Greg, you could try this method. If you mixed 15 kilos (plus or minus
1.5 kilos), the 10% difference is within the ballpark accuracy of the
Brongniart Formula, and you may be able to determine whether of not the
silica is missing or not.
You would need to know the total volume of the mix, in litres (3.8
litres per US gallon), and you probably will need to do two or three tries
of 200 mL samples, and average the result. This doesn't take long to do,
and it may settle the question.
good pots. BFN. Peace. Tom B.
Tom Buck )
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
| |
|
