Joseph Herbert on sun 22 oct 00
To Clayart
Materials – limestone and other topics
First thing, any of our correspondents who are using glaze recipes should
assume that any proportions, measurements, percentages, or other indications
of amount are given in weights, not volumes. There are very few volumetric
glaze type things, so unless there is a specific statement to the contrary,
the measurement are in weight.
About the composition of limestone.
Let’s start with the difference between rock names and mineral names. Rock
names do not, necessarily, carry information about the composition of the
material. The name can give you an idea but there are no requirements.
Limestone is a rock name. Mineral names DO give specific information about
composition. Calcite, a major constituent of limestone, is always calcium
carbonate. Calcite is a mineral name. Dolomite is both a mineral name and a
rock name. Dolomite, the mineral, is as Louis Katz described half magnesium
carbonate and half calcium carbonate. The composition of the rock Dolomite
is not so clear.
When talking of rocks, the only thing you can be sure of when you hear
limestone or dolomite is that they contain a fair amount of carbonate. Some
limestones are composed almost entirely of calcite, and therefore are almost
entirely calcium carbonate. Some limestones contain more or less amounts of
the mineral dolomite and contain both calcium and magnesium carbonate. Just
from the name of the rock, you cannot tell.
Limestone and dolomite powder as sold in large bags to ceramics people come
from mined (quarried) materials, rocks. The people in the quarry call the
rocks they work with by names that are convenient for them but which may not
mean the same thing to other people. The term whiting is a good example.
The material is named and used largely for its physical property of being
white. The fact that it is mostly calcium carbonate, calcite, allow a
chemical use of it also, as a glaze material.
The limestones we see in nature were deposited (mostly) in marine
environments in quiet water. Mostly, the particulate content of limestone
is not very large but that is not necessarily so. Some limestones contain a
fair amount of clay or even quartz. If the producer and his major customers
don’t care about the clay or quartz, the “whiting” may contain lots of it,
just as it comes from the ground.
There is a rock in Ohio called the Brassfield Limestone. In some parts of
Ohio it is composed of Very pure calcite. 99.99% calcium carbonate. In
other parts of Ohio, the same named rock, located in the same geologic
situation, has 2 to 5 percent dolomite (the mineral) in its composition.
This means that 1 to 2.5 % of the rock is Magnesium carbonate. This is
important to Portland cement makers. Every one who deals with this rock
calls it limestone where ever it occurs, even though that is not exactly
true.
We often refer to our glaze ingredients are “Chemicals.” This is true to
the extent that everything is a chemical. However, it gives the wrong
impression and may obscure that most of this stuff is just ground up rocks.
Specialized rocks to be sure but rocks. This means there are variations and
any other expectation is unrealistic.
There is a fairly easy way to determine the relative percentages of calcite
and dolomite in a powdered sample but it requires X-ray diffraction
equipment. If you know someone who has that capability, you can find the
truth about whiting.
Joseph Herbert
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