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clay specific gravity kink, anyone?

updated thu 5 feb 98


Jeff Lawrence on tue 3 feb 98


Being lazy and mathematically challenged, I asked my friend Norman the
physicist the following question:

How do you calculate the dry weight of material in a slurry of a known
specific gravity?

He produced a couple of equations which solve for the specific gravity.
However, these require that I know the specific gravity of clay. Submerging
a lump of clay in water to measure the weight and volume change looks
problematic (maybe it's not?)

Does anyone have a shop kink they'd be willing to share on measuring the
specific gravity of clay?

BTW, Norman said the easiest way was to weigh it beforehand, or to dry it
and weigh it then. True to his training, he only really got interested after
he established that it was not really necessary. True to my own kinkiness, I
am bothered by not being able to measure something that is right there in
front of me! Plus, Norman now refuses to accept a bowl I owe him unless I
glaze it with a slip I used his equations on.

Thanks in advance,


Jeff Lawrence
Sun Dagger Design
Rt 3 Box 220
Espanola, NM 87532
ph 505-753-5913

Gavin Stairs on wed 4 feb 98

At 01:34 PM 2/3/98 EST, Jeff wrote:
>However, these require that I know the specific gravity of clay. ...

>Does anyone have a shop kink they'd be willing to share on measuring the
>specific gravity of clay?

The problem is that what you want is the effective specific gravity in the
wet, hydrated condition(!).

The easiest way to get this is to measure it. You use the same formulae
your friend gave you, but you do it backwards. First you measure out a
batch of clay. If you want to mix dry, you weigh it out dry. If wet, you
weigh it out wet. The problem is to know how much water is there to begin
with, so totally dry clay works best. Mixes best, too. So now you know
how much clay you are using.

Then, measure out the water. You want enough water to make a loose slip.
Estimate this, and then add a bit more. Record the amount you actually
use, by weight. Now mix a slip, using all the clay and water. You now
have a slip in which you know exactly how much clay and how much water is
present. Check this by weighing the slip. It should equal the weight of
the clay and the water together. The amount you are off gives you an
estimate of how accurate your result will be.

Now measure the volume of the slip. You now have everything that the
formula requires, except the specific gravity of the clay, plus you have
the specific gravity of the slip. A little algebra will rearrange things
so you can calculate the specific gravity of the clay.

You may need to do this for each clay or body that you wish to use, as each
will give slightly different results. On the other hand, you may find that
they are all close enough that you can get away with one number for

Flocculants and deflocculant may change things, as they alter the way the
clay and water mix. You'll have to see for yourself. Hint: add some of
your favorite goop, and see if the volume of the slip has changed. If no,
then no problem.