|Monthly Tech-Tip |
Limestone forms by sedimentation, of coral and shells (biological limestone) or by the precipitation of calcium carbonate (as calcite and aragonite), in marine waters. It comes in many forms (e.g. chalk, coquina, fossiliferous, lithographic, oolitic, travertine, tufa). It can be crystalline, clastic, granular or massive. The production of calcium oxide by roasting limestone is one of the oldest chemical transformations done by man, it is the basis of mortar and cement (calcium oxide then reacts with carbon dioxide to regenerate as calcium carbonate).
Materials are considered limestone if they have 50%+ CaO (this comes close to the ideal formula of CaCO3). Manufacturers blend to produce a consistent product that is close to the ideal (with minimal impurities).
There are thousands of companies around the world that grind raw limestone into powders and aggregates. The powder is also called GCC (ground calcium carbonate), or just calcium carbonate. In ceramics, it is also commonly called "whiting".
GCC has countless applications, but in ceramics it is mainly used as a source of CaO (in glazes, frits, etc).
I gathered these downstream from the Oldman River dam in southern Alberta. My ultimate goal is to paint on underglaze designs, possibly a clear glaze over that, then fire them to cone 6. The Oldman River exits the Rocky mountains nearby, so these billions of rocks are not far from their original source. Upper right: The rocks as I gathered them. Lower left: After firing to cone 04 (about 1900F). Lower right: After I poured water on them! Seven disintegrated in front of my eyes. With sound effects, even popping! Generating high heat and steam in the process. That means they are all limestone or dolostone. The firing drove off the carbonate. But the calcium sulphate wants it back and welcomes it with a fanfare. #5 and #10 cracked badly. The unfractured ones appear to be iron-bearing silt and sand stones, they should withstand considerably higher temperatures.
Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.
|Materials||Camadil 95 Dolomite|
|Oxides||CaO - Calcium Oxide, Calcia|
Mineralogy Data for Calcite
Limestone at Wikipedia
What is GCC at specialtyminerals.com