Stuart Altmann on sun 26 apr 98
Janet,
So, you slipped when mixing slip: added too much water. What to do?
First, how much water did you use? If you don't know, begin by using
Brongniart's formula to find out. For example, if 100 cc of your slip
weighs 150 g, then the density of the slip is 1500 g/liter (i.e., relative
density or specific gravity is 1.5). If the dry matter in your slip has a
density of 2500 g/liter (the approximate density of non-lead stoneware
glazes), then Brongniart's formula tells you that your 100 cc of slip
contains 83.3 g. dry matter and 66.7 g. water. That is, the water that you
added weighed 66.7/83.3 = 80% as much as the dry matter. So if, say, you
had 14 kg dry matter in your batch--a common amount for a 5-gal tub--then
you must have added 11.2 kg water.
Second, how much water *should* you have used to get your desired s.g. of
1.75? To tell how much dry matter and how much water would give s.g.=1.75
(and thus what the percent water should be), use Brongniart's formula for a
hypothetical one liter sample, here still assuming a density of 2500 g/l
for the dry matter:
g dry matter per liter of slip = (density of slip, g/l - 1000)5/3
= (1750 - 1000)5/3 = 1250 g/l;
g water per liter of slip = 1750 - 1250 = 500 g/l.
So, water should be 500/1250 = 40% of dry matter to get s.g. of 1.75. For
your 14 kg batch of dry matter, that would take 5.6 kg water.
The difference between the water you used (11.2 kg) and what you should
have used (5.6 kg) is 5.6 kg extra water. That is, in this example, you
used twice as much water as you should have.
Finally, you can cope with the extra 5.6 kg of water in either of two
ways. Perhaps the easiest is to let your slip settle, then decant 5.6
liters of water off the top. Alternatively, you can add extra dry matter
to compensate for the extra water. In this case, the amount of additional
dry matter needed is obvious: you used twice as much water as your recipe
calls for so you must use twice as much dry matter, that is, you need to
add another 14 kg dry matter to your too-thin slip. But suppose the amount
of extra dry matter needed is not so obvious at this point. We can
calculate the amount by asking, what amount of extra dry matter (x) times
40% would equal the amount of extra water. Thus, in the above example, we
write 0.4x = 5.6 kg, and so x = 14 kg. In practice, I would hold back about
5% of the total water and 5% of the total dry matter, then use this
held-back material to fine-tune the slip's density.
Hope this helps,
Stuart Altmann
Fay & Ralph Loewenthal on mon 27 apr 98
Stuart, are you sure about those calculationsof Brogniart's
formula? Taking a purely practical viewpoint 40% of water
to dry powder will give you a very thick porridge of glaze.
We usually mix in about 1 litre of water to 1 Kg of dry glaze
powder, and sometimes even more water. The way Fay
usually tests is by sticking her hand in dissolved, sieved
glaze and if she can just see her skin through the glaze it
is fine. Ralph in a coolish bright day in PE SA
Stuart Altmann on wed 29 apr 98
"Stuart, are you sure about those calculations?...40% of water to dry powder
will give you a very thick porridge of glaze..." Ralph Lowenthal
Ralph,
I will check my calculations--to err is human--but Janet was making a slip,
not making a glaze, though whether for brushing, casting, or whatever, she
doesn't say. She did add a deflocculant to the slip, and that would reduce
its viscosity without changing the density.
Stuart Altmann
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