Michael McDowell on mon 15 apr 96
Following this discussion on Clayart I was at first baffled by the parts
referring to changes in apparent Specific Gravity when electrylotes are added.
Then I realized that this was in reference to using a hydrometer in slips, whigh
generally run much thicker in consistency than glazes. I don't work with casting
slips, but I do use a hydrometer in much the same way Vince relates for my
glazes. In principle there should be no difference between the readings a
floating hydrometer gives Vs. the weighing method, but obviously , now, there is
a problem if the mixture is so thick that instead of floating, the hydrometer
just sticks in the mix wherever you put it.
Some of my thickest glazes will begin to show this kind of problem, & I have
worked out a simple technique that gives me consistent, if not accurate,
readings with them. The trick is to lubricate the float so that it doesn't have
difficulty moving up & down through the mix to find its true floating level.
What I do is dip the hydometer first in water, then plunge it into the mix to be
measured slightly below what I think will be the true reading. Then I pound on
the outside of the glaze bucket lightly to agitate the mix a bit & let the
hydrometer rise to a stable level. The thin coating of water on the glass
encourages glaze to fall away from the tube as it rises above the level of the
mix so it is posible to read the instrument.
There is no one correct Specific gravity for a glaze. It is strictly a matter of
trial and error & error & trial to determine for each glaze & method of
application what the target reading should be. Keep notes. Once I have found my
desired S. G. for a new glaze, I put a tag on the bucket with the target reading
so its right there in view whenever I am mixing & applying the glaze. For me,
its an indispensable tool. One note of caution, the glass tubes can be very
breakable. The other day I was holding the tube in one hand & wiping the bottom
with a sponge when I inadvertently snapped the tube in two, just with the
pressure of holding it! The new one I had sent UPS from my supplier ($16.95 +
frt.) seems to be a little thicker glass, but I will be more careful now too.
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