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glaze hydrometers and questions...

updated mon 30 dec 02


Von Allen on fri 27 dec 02

Hello everyone, I've spent nearly an hour catching up on posts. I am =
wondering if anyone knows what a glaze hydrometer is and where to get =
I have a recipe for terra sig.that swears by the instrument.
Also, I was reading posts that said that chrome could be absorbed thru =
the skin. I am curious about what chemicals can go thru the skin? I =
called the poison center some years ago and was informed that chemicals =
do not go thru skin, only sores or wounds in the skin. Were these people =
If stuff goes thru skin, how can I put my hands in bleach? Water does =
not go thru my skin, so how could anything else go thru it? Just =
wondering, I'm perplexed on this one. Also concerned as I see students =
getting glaze on their hands. Should we all be wearing surgical gloves? =
Do chemicals go thru gloves? Inquiring minds want to know..

Vince Pitelka on sat 28 dec 02

Von wrote:
"Hello everyone, I've spent nearly an hour catching up on posts. I am
wondering if anyone knows what a glaze hydrometer is and where to get one.
I have a recipe for terra sig.that swears by the instrument.
Also, I was reading posts that said that chrome could be absorbed thru the
skin. I am curious about what chemicals can go thru the skin"

Von -
Chemicals that are water soluble or oil soluble can go through the skin,
although Edouard Bastarache could give you a much more definitive answer
there. Chrome oxide is not water soluble, and is fairly coarse particle
size, and cannot be absorbed through the skin. None of the standard ceramic
oxides (chrome oxide, cobalt carbonate, cobalt oxide, manganese dioxide, red
iron oxide, black iron oxide, copper carbonate, copper oxide, rutile) can be
absorbed through the skin. It is only common sense to keep them away from
an open sore or wound.

It is best not to refer to a "glaze hydrometer" because they do not work
worth a damn for glazes. They work great for very thin suspensions, like a
terra sig. For the best deal on a quality hydrometer, go to and enter "hydrometers" in the search box. You will
find a wide range of reasonably-priced hydrometers for every range. They
have a number intended for "heavy suspensions" (in chemistry terms, a terra
sig is a heavy suspension) that measure in the 1.00 to 2.00 range, but keep
in mind that in the upper end of this range hydrometers are extremely
inaccurate. They are great for doing terra sigillata, and if that is what
you want then you should get the one they sell which measures from 1.000 to
1.225. That covers the full range you would need for terra sig, and with a
lot more precision than the one which covers the full range from 1.00 to
Good luck -
- Vince

Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home -
Work -
615/597-6801 ext. 111, fax 615/597-6803

Lily Krakowski on sat 28 dec 02

Inquiring Minds ought to read:

Hydrometers are available at Farm Supply houses, places that sell
educational toys, and so on. Actually the gismo that tests your
antifreeze--do you have enough-- is one. You also can make your own: see
Pottery Making Illustrated July/August 2002, p 36.

Poison Control: These places are wonderful, but in my experience do much
better with emergency: the baby just swallowed.... than with the long term
effects of bad stuff. OF COURSE STUFF CAN GO THROUGH THE SKIN!!!! How
would things like "patches" with nicotine, nitroglycerine, birth-control
medication work otherwise? Go out and buy yourself one of the several books
on safety in the studio and READ IT. The Potters' Shop has several and
will recommend what is best for you. Safety/Health books are the one thing
I urge you to buy brfand-new, latest edition, as more info. comes on line.

Also: The clay magazines are excellent on keeping us informed about health

Rubber gloves are highly recommended. If nothing else they protect the skin
from the harsh effects some glazes can have on the skin. That way when
someone kisses your hand, or nibbles the inside of your wrist s/he won't
sandpaper h/er/is lips.


WHC228@AOL.COM on sat 28 dec 02

Hygrometers for testing the specific gravity of glazes are usually not very
accurate. The problem lies in the fact that glaze is very viscous. You may
put the hygrometer into to glaze and get many different readings.
I routinely check the specific gravity of my glazes with my triple beam
I use a narrow necked ball flask that has a line scratched into the neck. The
small neck is necessary so that any error is minimized.
To set this up you should weigh the flask empty, and make note of the weight.
Second step is to weigh it with water up to the line.
Third is to subtract the weight of the flask from the water to determine the
weight of the water.
When weighing the glaze fill it to the line and weigh the glaze. Subtract the
weight of the flask, and you will have the weight of the glaze. The ratio
between the weight of the glaze and the water is the specific gravity.
The thing to do with these numbers is to develop a range of numbers in the
specific gravity range that you plan to work in, and make a little chart to
keep near the flask, so that you can take the weight of the glaze and read
the chart when you need to know the SG.
It sounds like the long way around to do it this way, but it works perfectly
every time.
It is useful for setting your glazes and also for setting deflocculated slip.

Martin Howard on sat 28 dec 02

There is a hydrometer made in the UK.
But its measurements are in degrees Baume'.
I have posted the more normal equivalents in a message some years ago.

It is best to measure the density of the liquid by weighing it and
subtracting the weight of the vessel and comparing the weight of the liquid
with the same volume of water. All a hydrometer does is measure the density
of the liquid, but you want it in normal form, not Baume' degrees to make
your terra sigillata.

Water goes through the skin, both ways.
Urine and all the constituents of urine (see for a fairly full list of the
found constituents or urine)

Whatever we put our hands in when doing pottery is liable to go through the
skin if it is find enough and molecular structure permits.

So be careful and wear plastic gloves when necessary.

I would certainly not pick up or handle any of the mineral oxides which we
use in slips and glazes with the bare hand. There is never any need to do
When it diluted in slip or glaze and some gets on the hand, don't worry.
But wash it off as soon as possible.
It is a matter of common sense as well as chemistry.

Martin Howard
Webbs Cottage Pottery
Woolpits Road, Great Saling
01371 850 423
Updated 8th December 2002

Gavin Stairs on sat 28 dec 02

At 10:25 AM 28/12/2002 -0500, you wrote:
>Hygrometers for testing the specific gravity of glazes are usually not very
>accurate. The problem lies in the fact that glaze is very viscous.

Not viscosity, but thixotropy. That means that a thick glaze will support
some level of shear, which can distort the density reading. The other
thing to beware of is settling. If there is a settling out of some part of
the glaze, even if the top layer is not clear water, you can get a wrong

A much better way to measure glaze density is to use a cup or flask of
standard volume, and weigh the flask. The procedure is to get a container,
perhaps a plastic cup, that you can weigh on your most accurate
scale. Weigh it empty and then with water in it up to a mark. Then weigh
it with the glaze, well mixed, and up to the same mark. You don't know
what the volume is, but that doesn't matter. Calculate as
follows. Subtract the empty container weight from the
water+container. That's the water weight. Then subtract the container
weight from the glaze+container weight. That's the glaze weight. Record
the glaze weight and the container weight for future measurements. Then
divide the glaze weight by the water weight. That number is the specific
gravity, which is numerically equal to the density of the glaze in g/mL, or
g/cc, since the density of water is 1g/mL. The accuracy of this method
depends on the homogeneity of the glaze (how well mixed it is), and
variables of the filling (Is the container level when the liquid is at the
mark? Is the level at exactly the same place as when you weighed the
water?), with minor contributions by temperature, air bubbles, etc. The
precision depends on the least count of the scale.

Many potters use a plastic cup or jug to do this measurement. If you are
making a large batch of glaze, and can dip a plastic cup into it, you can
usually get close enough by simply weighing the cup full of glaze and
dividing by the weight with water. If the cup holds some volume like 1/4
or 1/5 L, then the division can be done in your head, making this very
quick. The cup's own weight may be negligible for a rough
measurement. However, if you use a ceramic, metal or glass cup, you will
probably have to subtract the container weight.


J. B. Clauson on sat 28 dec 02

A hydrometer is an instrument that measures the density of a solution. The
simplest is a sealed glass tube weighted at one end with density gradients
pronted on the side. The one I have I purchased many years ago at a pet
store that specialized in salt water tropical fish. You need to measure the
density of your salt water to maintain the proper balance for the health of
your fish. I've never used it for glaze. My teachers preferred the "stick
your hand in it and see how it adheres" method which is what I use.

Dean Walker on sat 28 dec 02

If birth control can be accomplished with a patch absorbed through the
skin and nicotine patch can help one to stop smoking, I know things must get
into the blood through the skin.