Bill Aycock on sat 20 apr 96
After a brief exchange of off-list notes with Don Kopyscinski, I decided to
run a set of "horrible example" hydrometer tests, using --- what else---
My only hydrometer just now is an old "heavy liquids" glass instrument
calibrated in degrees Baume'. This scale is convertible to Specific Gravity
with a little arithmatic. Its major shortcoming is that it wont measure
any SG below about 1.38, which puts it outside the range where Vince works,
with his Terra Sig.
My tests proved that Don and those of his way of thinking are---
*** Absolutely Right ***
A hydrometer is practically impossible to use for any mix with a Sg above
(a guess) about
1.3, when materials like glazes and slips are involved. I was able to do
some surprising things with the readings by using things like an ear
syringe, vinegar, Epsom salts, Sodium silicate, and Ajax("with real lemon")
I was also able to get a sloppy approximation of the true value ( as
measured by weighing both water and samples in the same container, and
correcting for container weight), but at a cost in time and detailed
attention that was ludicrous.
I selected a waste mix that I knew contained some Gerstley Borate, and mixed
it well, followed by 80 mesh screening. The container I used to hold the
stuff while I made readings was a plastic pint milk container, about 2.5
in in diameter, and tall enough to let the hydrometer work at its full
"stroke". I used this small container so I could throw each sample away and
start with fresh stuff for each trial.
1. With a mix having an SG of about 1.5, I could get readings of 1.38 to
1.73, depending on how I inserted the gadget. At either extreme, I could
get the hydrometer to move toward the right value by vibrating the
container. Note: toward, but not TO the right reading.
2. By adding water (and mixing pretty well) to an SG of about 1.37 (near the
limit of the scale), I reduced the range between the upper and lower
readings. Adding vinegar brought the range of readings right back up again.
3. One cute illustration of the problem came when I was taking out a little
glaze to make room to add an electrolite. I could raise and lower the
level of the liquid in the jar without the hydrometer moving,( when I used a
syringe to take it out and not disturb the Hydrometer).!
4. The electrolites did just as expected; "Thinning" reduced the error, and
"Thickening" made it worse. (I used different samples for these measurements. )
5. One worthless surprise- on a whim, I added some liquid Ajax and mixed it
in with an ear syringe-- this reduced the viscosity a lot, and the
hydrometer readings improved, but the improvement was not worth much-- the
foam on top made it very hard to read.
6. After each set of hydrometer measurements for the effect of electrolite
addition, I weighted the filled container to be sure I was not making a
significant SG change. Each sample was poured out, back into the waste
So--- I found another use for waste glaze, and proved that, while a
hydrometer is a great gadget at some levels of SG, at the level used for
most glazes and slips, it is apt to be very missleading. In fact, I think
it is worse than no reading at all. A slight amount of experience will give
you better guesses ( when working at high SG levels) than the range of
readings I was able to get . Weighing a known volume is both easy and accurate.
But I still have the waste glaze, improved (?) with the addition of various
reagents of opposing effect.
Bill, after a rainy inside day on Persimmon Hill.