Steve Mills on mon 30 dec 02
I used a home made Hydrometer for years when I was in full time
production; a vital piece of kit. My trick was to wet it before use. So
long as I ALWAYS did that it was consistent (and so were my glazed
wares). It was made out of a test tube, weighted with fishing shot held
in place with wax, with a home made (arbitrary) scale super-glued
inside, and sealed with a cork
Steve
Bath
UK
In message , Roger Graham writes
>I've been following the posts re using a hydrometer for measuring the
>specific gravity of a glaze. Not everybody is enthusiastic about
>hydrometers. They don't sink to the same depth every time, by reason of
>viscosity or thixotropy in the glaze. They give unreliable results if you
>don't stir the glaze (surprise!). They have fleas. They embarrass you at
>parties.
--
Steve Mills
Bath
UK
Roger Graham on mon 30 dec 02
I've been following the posts re using a hydrometer for measuring the
specific gravity of a glaze. Not everybody is enthusiastic about
hydrometers. They don't sink to the same depth every time, by reason of
viscosity or thixotropy in the glaze. They give unreliable results if you
don't stir the glaze (surprise!). They have fleas. They embarrass you at
parties.
All of this has disturbed me. I've been using a home-made hydrometer with
every confidence for years. Shared the idea with umpteen other potters. Made
a dozen or more from time to time, to be given as freebies. Posted them to
faraway places, even overseas. Could I have been so wrong, all these years?
It seemed a good idea to check my trusty hydrometer against the conventional
"relative density bottle" method. I found a stubby beer bottle, just right
for the job. Weighed it, clean, dry and empty (173 grams) and again filled
completely with water (570 grams). So it must hold (570-173) = 397 grams of
water, and its volume must be 397 cubic centimetres, or 397 millilitres.
Even the mathematically challenged can keep up with this.
Now for the moment of truth. I chose a bucket of glaze, talc white. Stirred
it all up in the usual way. Floated the hydrometer, and gave the bucket a
whack so the hydrometer settled to the true depth. Hydrometer said 1.4
Filled the stubby beer bottle with well stirred glaze. Weighed it again (738
grams). So the weight of glaze was (738-173) = 565 grams,and the relative
density of the glaze must be (565/397) or 1.42
The score is hydrometer 1, unbelievers nil.
I did it all again with the bucket of Indian Red glaze. Won't bore you with
the figures, but the results were hydrometer 1.32, relative density bottle
1.29 That's a difference of about 2 percent.
The score is hydrometer 2, unbelievers nil.
Did it again with the bucket of Speckle Matt glaze, chosen because it's so
thin and watery. Results, hydrometer 1.28, other method 1.29
Once more, this time with a denser more creamy glaze. Hydrometer 1.48, other
method 1.49
Them's the figures. So hear me when I speak in favour of the humble
hydrometer. Not as unreliable as some posts have made out. I use a
hydrometer every time, when making up a
new batch of glaze. And whenever revisiting a bucket of glaze that hasn't
been used for a while.
Looking to make or buy a hydrometer? For a start, you can't use the kind of
hydrometer they sell for testing battery acid. The range of specific gravity
values for a car battery goes
from about 1.1 (dead flat) to 1.27 (full charge). So the battery hydrometer
scale only goes as high as about 1.3 You'll need a range from about 1.1 up
to say 1.7
Hydrometers aren't a big success for creamy slip or very viscous clinging
glaze, as several posts have pointed out. Can't argue with that. But for all
the 20 or so glazes in use here, a hydrometer works OK. If the glaze is thin
and watery, the hydrometer sinks to its own level, the same level every
time, without being dragged or retarded by the liquid. If the glaze is a bit
more thick and creamy, the hydrometer needs some encouragement to make it
sink the last few millimetres to its true level. Tapping the side of the
container to agitate the liquid seems to do the trick.
You don't need to buy a fancy expensive instrument. Making and calibrating
your own is no big deal. I shared the design of our home-brewed one with Des
Howard, who scanned the information into a pdf file on his website. It
gives
all the detail you need, starting with a McDonalds drinking straw. Visit his
website and you can download and print a one-page instruction sheet.
Des Howard's website is at http://www.luepottery.hwy.com.au
Follow the instructions and you don't even need to calculate where to put
the marks on the hydrometer stem. If you've made a print of the instruction
page, there's a calibration chart showing what to do. Just line up the straw
against the chart, and it shows where to put the marks.
Try it. Hydrometers aren't cheap and nasty. Just cheap.
Roger Graham, near Gerringong, Australia
http://member.optusnet.com.au/~rogergraham
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