MAKING CHALK

TEXT PROVIDED BY
BINNEY & SMITH

There are two ways of making chalk; in one the chalk is molded, in the other it is pressed and extruded, like toothpaste from a tube. Extruded chalk, whether white or specifically coloured, is
most commonly used on chalkboards in school, as it tends to be dense and "dustless".

To produce extrued chalk, the makers first stir together, by machine, several white powders which look like flour. The most important of these is calcium carbonate and water-washed clay. If the chalk is to be coloured, the particular colour is likewise  added in the form of dry, finely ground pigments.  After all of the powders are well mixed, they are transferred to another large machine where a liquid "binder" is poured in to hold the dry particles together. If you have ever added milk to dry ingredients in order to bind them together for batter or dough you will have a good idea of the process here.

After the powders and their binder have swished around for a certain length of time, the chalk forms a kind of dough which looks like many small balls about the size of marbles. These little dough balls are then machine-pressed into a large, long shape, just like a solid cylinder. This shape is called a cartridge. As a result of this pressing, air is forced out of the dough, and the moist particles in the dough are very closely bound to one another causing the chalk to be heavy and smoth textured.

The cartridge, which is still damp and pliable, is then inserted into another machine called an extrusion press, where it is forced through a small tube. As the long rope of wet chalk comes out, an automatic slicer cuts it into many pieces. These pieces roll down to a tray, and are ready to be cut again into regular size sticks of chalk as we kow them. Since the pieces are still quite moist, they must be dried in large ovens, called kilns, before they become hard enough to be packed.

If you remember, extruded chalk was described as "dustless". Now, we all that chalk, being dry, is naturally dusty. In the case of extruded chalk, however, the dust particles are weighted to fall straight down instead of flying through the air to make you sneeze. In other words, the dust in "dustless" chalk is chemically controlled.

The moulded method, on the other hand, produces chalk which is softer than the extruded chalk, and is NOT dustless. Dry pigment and water are first mixed in preparation for the addition of plaster of paris. Because plaster begins to set quickly, this process must be carefully timed and supervised. Before the mixture gets too thick, it is poured into a molding machine with many holes that are the exact size and shape of the finished pieces of chalk. Excess chalk "batter" is scraped off the tops of the moulds with a wooden paddel, and after a setting of five to eight minutes, depending upon the particular colour being manufactured, the sticks of chalk are popped out onto a wooden tray which is then stacked in a large drying rack. Like the extruded chalks, these are kiln-dried and packaged.

Both molded and extruded chalks are manufactured in a variety of colours and shapes. There are round sticks, long square ones, and even hemispheres, and their uses are varied. Chalks made for school and home are different from those needed in an art studio or factory; and just as in a family, no two members are alike even though they may resemble one another. Although the chalk makers at Binney & Smit produce many types of chalk, all types are either extruded or moulded.

Look closely at the next piece of chalk you use. Can you tell which type it is? With a bit of practice in comparing them, you should become an expert in distinguishing an extruded stick (hard and dense), from a moulded stick (soft and porous),  and that's more than MOST chalk users can say for themselves!