YOU ASKED: May I ask what you mean by calcium carbonate? I read in your description of “calcium carbonate” you compare it to chalk. This confuses me because chalk is caso4•(H2O) while calcium carbonate is caco3.
KATHY ANSWERED: What we call “chalk” in the US is Calcium Carbonate – CaC03, a powdery white material. The formula that you reference is Calcium Sulfate and it may also be called “chalk” but in the US, we commonly refer to it as “gypsum”. Calcium carbonate is used to mark soccer fields, as a dietary calcium supplement, an ingredient in antacids, and building materials! It comes in different preparations – from food grade to industrial grade depending on the end use. We also call Calcium Carbonate “Limestone”, and if you have limestone deposits in your area, chances are that you also have calcium rich water. Here is an article in English about the chemical differences between Calcium Carbonate and Gypsum.
To make it even more confusing, the art and drawing material that is called “chalk” is actually often made of gypsum! We use Calcium Carbonate in the dye process but I have not used Calcium Sulfate so I don’t know how it would react. I hope this helps you!
FUN FACT: Dyes that love Calcium Carbonate
Calcium carbonate is a useful ingredient in natural dyeing in that it can used to bloom colors that might appear a bit muted in the dye bath. There are several dyes that benefit from the addition of a small amount of chalk. In most cases, just a small amount is needed, and we usually add about 1% on wof if we are using it in a dye recipe. Mix it in a cup or jar with hot water and add to the dye bath before adding fibers.
Madder Extract and Madder Root – madder loves calcium and often a small amount (1% or less on the weight of fiber) added to the dye bath will yield deeper, redder shades. Of course, if you already have calcium rich water, then you should already see lovely reds in your dye baths.
Cutch Extract – cutch develops darker russet brown tones with calcium. We find that cutch takes a long time to develop, so we will often combine it with a small amount of calcium carbonate and soda ash (1% or less on the weight of fiber) to coax the rich red-brown shades from the dye.
Weld Flowers and weld extract – weld loves calcium carbonate and develops a deeper bright yellow with calcium added to the dye bath. Calcium carbonate can also help develop other yellow dyes, so if you are experiencing a rather pale and weak yellow shade, try removing the fibers, adding a small amount of calcium carbonate and then adding the fibers back and see if the color deepens.
Logwood – logwood also develops a richer purple shade with calcium carbonate, especially if you have soft water and experience that logwood can be very light, brownish or seems to need a boost.