Calcium sulfate an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: This reaction is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth, an
Chalk talk (1842-1905). Today chalk talks are still regularly used as a method of preaching scripture visually, by evangelists that have artistic ability. A book by Jeanne Bridge, "Chalk Talk for Catechists," gives us a useful introduction to a catechetical method, where we see (what?) is an increasing interest among all sectors of people. Basically, it talks about "speaking the message" by illustrating with simple chalk drawings, along with the talk, that highlight the main points of the lesson. The word which is predominant is at a lead together with the word [this phrase says nothing], so that the word and the