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## specific gravity and density

### Stuart Altmann on tue 25 nov 97

First, the take-home messages for potters. (1) Specific gravity, if
measured relative to water, is the same as density, except that the former
is dimensionless whereas density is in units of mass per volume. Also, what
the Hamers, in their "Potter's Dictionary," call 'relative density' is just
density. (2) If you weigh a pint of glaze in grams, then divide by grams of
water per pint, you have included an unnecessary extra step. Instead, weigh
a liter of glaze, in kilos. That's its density (its specific gravity)!
Also, you will get somewhat more accurate results by weighing the larger
volume of liquid. (3) Forget about degrees Twaddell, if indeed you ever
knew about them. (They are described in the Hamers' volume.)

Now for the details. Specific gravity and density are the same, though that
may not be apparent from their definitions. Specific gravity is the ratio
between the mass ("weight") of a substance and that of the same volume of a
standard substance (usually water for liquids and solids, air for gases),
measured under standard conditions. Density is simply the ratio of mass to
volume. In the SI metric system, density is in units of kilograms per cubic
meter. Since a liter is defined as the volume occupied by 1 kg of pure
water at its maximum density (4 C) and a liter is also a cubic meter, the
density of water is 1 and the definition of density is equivalent to kg per
liter. That is, if you calculate a glaze's density by weighing out a liter
of it, in kilograms, then divide by 1 kg (the mass of the same volume of
water), the division by 1 does not change the value, right?

Degrees Twaddell? Our glazes have densities between 1.0 and 2.0, e.g.
glazes for dipping are usually diluted to about 1.6. Some people find a
range from 1 to 2 to be "too compressed," and so degrees Twaddell converts
density into values that range from 0 to 200. But the calculations are both
cumbersome and unnecessary. When my triple beam balance tells me that my
glaze has a density of 1.27, I know that I'm still a fair distance from,
say, 1.60 and if I'm mixing up a largish batch of glaze, the amount of water
that I need to add is not minute. Don't let that decimal point in there
fool you. Its location just depends on our scale of measurement, e.g.
kilograms rather than grams.

Well, we have the same psychological problem with proportions: we use
percentages instead. Okay, so if you are bothered by feeling that a density
range of 1.00 to 2.00 is compressed, you can multiply all your density
values by 100, giving you a new unit, and glazes now range from 100 to 200
units. That's a lot easier than the conversion to degrees Twaddell, but
I'll bet that after you do it a few times you will realize that moving the
decimal over two places doesn't really accomplish anything and you might as
well go with density as defined.

One more item. You may have noticed that I wrote "weight" in quotation
marks above, where I really meant mass. Although weight and mass are
commonly confused, they are not the same. Weight is a measure of force and
so is measured in units such as newtons, dynes, or poundals. The metric
unit of weight is the newton, defined as one kilogram meter per second-
squared, whereas the unit of mass is simply one kilogram. Are they
numerically the same? No. You weigh more in an ascending elevator and less
in a descending elevator or on a mountain, and in outer space you are
weightless, though your mass does not change, unless, say, you eat
or...well, you get the idea.

Stuart Altmann

### Denis Whitfield on wed 26 nov 97

Stuart,

Oh dear!

There are 1000 litres (liters) in a cubic metre (meter).

A cubic metre of water "weighs" 1000 kg (ie 1 tonne). (It actually weighs
around 9800 Newtons on earth).

The density of water is 1000 kg per cubic metre.

The specific gravity of water is 1

Denis

At 08:09 25/11/97 EST, you wrote:
>----------------------------Original message----------------------------
>
>Now for the details. Specific gravity and density are the same, though that
>may not be apparent from their definitions. Specific gravity is the ratio
>between the mass ("weight") of a substance and that of the same volume of a
>standard substance (usually water for liquids and solids, air for gases),
>measured under standard conditions. Density is simply the ratio of mass to
>volume. In the SI metric system, density is in units of kilograms per cubic
>meter. Since a liter is defined as the volume occupied by 1 kg of pure
>water at its maximum density (4 C) and a liter is also a cubic meter, the
>density of water is 1 and the definition of density is equivalent to kg per
>liter. That is, if you calculate a glaze's density by weighing out a liter
>of it, in kilograms, then divide by 1 kg (the mass of the same volume of
>water), the division by 1 does not change the value, right?
>
>
>
>Stuart Altmann
>
>
****************************************************************************
DR DENIS WHITFIELD
Senior Lecturer
Coodinator BA(VA)
Co Director, Centre for Ceramic Research, Design and Production
Department of Visual and Peforming Arts
University of Western Sydney, Macarthur
PO Box 555
CAMPBELLTOWN NSW 2560
AUSTRALIA
email: d.whitfield@uws.edu.au
phone: 02 97726345 international: xxx 61 2 97726345
fax: 02 97723244 international: xxx 61 2 97723244
****************************************************************************

### Terrance Lazaroff on thu 27 nov 97

I am confused. Are you saying that you must add water to the glaze batch
that has a density of 1.27 to arrive at 1.60. I always thought that adding
water reduces the density number. IE. If I have a density of 1.5 and I add
water I will end up with a number smaller than what I started out with.

I could be wrong. Or I could be confused as to what you are saying.

Terrance