Ivor J.Townshend on mon 30 aug 99
A week or so ago I asked for help with reading my hydrometer which is
scaled from 40 to 60 degrees Baume not in grams per millilitre or the
specific gravity ratio. I have had several replies both on and off
Clayart, but due to the pressure of a summer Raku Workshop, I have
only managed to acknowledge these rather than reply properly.
Many thanks Bill, Fred, Joel and Tom... and anyone I missed out - I'm
still a little behind with reading Clayart. The non-ceramic sources I
had found used non-comparable rules and my results were (predictably)
The correct rule for converting Baume to SG is Specific Gravity
equals 145 divided by X where X is the quantity (145 minus Baume)
(I hope this still makes sense after it has been chewed up by the
There were two sources for this information which helps to establish
Eshbach, Handbook of Engineering Fundamentals 3rd edition p167 and
Handbook of Chemistry and Physics (Chemical Rubber Handbook) 31st
The results are
Baume equals SG
There was also a strong caution from many correspondents against
using a hydrometer for glazes citing work by Bill Aycock reported on
Clayart before I joined. This work showed that inaccurate results
can be obtained.
My view is that Specific Gravity measurement is a skilled
process even with a clear non-viscous liquid and care is required.
It should not be attempted on viscous materials or those glazes prone
to settling or stratification unless the observer is prepared to take
steps to eliminate the likely errors. Multiple readings will help
and it is necessary to allow time for stabilisation in even slightly
viscous fluids. If the material is also prone to settlement then
this is very problematic. The glaze will stratify or separate whilst
you wait for the hydrometer to settle. Clearly, there will be many
cases where this method is inappropriate, but perhaps not all.
If a weighed container is available - together with an accurate
balance - then it is often simpler to check the weight of this
standard measured volume of glaze than to struggle with a hydrometer.
The density of the glaze is then adjusted until a sampled volume
weighs the same as when the glaze was first made up.
A final caution do keep the balance clean and use check weights on a
Thanks to the moderators of CLAYART for providing such a wonderful
forum. I am impressed with the speed and accuracy of the information
given by correspondents.
Ivor (In Macclesfield, just up the road from Josiah
Wedgwood's original pottery)