Where Do School Slates Come From?
Slates such as are used in school and as roofing material are formed of clay, which has been hardened under pressure and heat. When this occurs it does so because a number of layers of clay, one on top of the other, have at sometime been subjected to great heat and pressure within the earth with the result that the clay is pressed into very thick layers and changed in color by the heat and becomes hard. There are many kinds of slate. Some of the slate, as found in slate mines, is used to make roofs over buildings and for this purpose they are cut to shapes very much like wooden shingles. They are easily broken, however, as slate is very brittle.
Slate is used in many other ways besides for roofs and school slates. Sometimes it is made into slate pencils but, since paper has become so cheap, comparatively few slate pencils are used in the school room today.