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hydrometers and glazes

updated fri 14 nov 97


Joseph Herbert on thu 13 nov 97

Hydrometers can measure the density of liquids using the famous Archimedean
"Eureka" principle. This is famous classical physics from the cradle of
western civilization. Also from that same cradle comes the admonition to
check your assumptions. Glazes and slips are suspensions or slurries NOT
liquids - so why are you using a hydrometer at all. Weighing is good, I
imagine Jonathan Kaplan keeps better records than any of us, and good records
are really what make measuring work. It s the comparison of the ongoing
results of a consistent method that really tell you anything. A stick with a
weight on the end and marks on the side will work, don t loose the stick. A
third method that no one uses is to time the flow of a known amount of
suspension through a standard funnel. It has the advantage that most funnels
are unbreakable.

The need for a hydrometer is, for most situations, an illusion. Most of the
troubles with glazes are caused by incomplete mixing - so materials are
removed selectively over time, improper application - too thick or too thin -
again mixing can play a part. Most of the experiences I have had are in
shared space situations and the right amount of water in a glaze suspension
was way down on the list of possible problems. If you have control of your
own space, spend your time worrying about weighing and cleaning. The water
will probably take care of it self. If you are in a shared studio, someone
else will do something to make the water the least of your worries. Like I
said, an illusion.

Joseph Herbert