GYPSUM DEiPOSITS OF NEW YORK 8/ 



The composition of the plaster, other things being equal, is an 

 index of its setting properties. A pure plaster of paris of normai 

 fineness when mixed with water will harden in about six minutes. 

 This is known as the initial set.-^' Impure piasters, on the other 

 hand, may require an hour or more to harden. 



Addition of retarders. Plasters intended for wall and other 

 structural purposes must be slow setting to avoid difficulty in 

 manipulation. If this is not a natural property, which may be 

 found in the impure sorts, it is necessary to induce slow setting by 

 the addition of some foreign material. As a matter of fact practi- 

 cally all wall plasters, however impure, require treatment with a 

 '' retarder " by which the time of setting is proilonged to from two 

 to four or five hours, according to need. 



The retarders employed by manufacturers O'f wall plasters in- 

 clude such materials as glue, glycerine, chemically prepared hair, 

 slaked lime, sawdust, and the tankage from packing houses. Most 

 manufacturers have a preference for some particular material, the 

 nature of which, as well as the proportions used, is generally care- 

 fully guarded. There are also several patented preparations on the 

 market. The effect of the retarder is probably tO' decrease th^ 

 solubility of the plaster and thus to extend the period of hydration 

 and recrystallization. 



The retarder is added to the cool ground plaster in amount vary- 

 ing from 2 to 15 pounds a ton and is thoroughly incorporated by 

 the use of a mixing machine. 



Wall plasters also contain some fiber — hair, wood or asbestos — 

 which is added before mixing. From i to 3 pounds of hair to a 

 ton of plaster is the general proportion. The hair nuist be pre- 

 viously teased out by a picker. The wood fiber is made from a 

 soft wood Hke poplar, willow or basswood. The wood, cut into^ 20- 

 inch lengths, is run between two revolving toothed cylinders which 

 rapidly shred it. The mixing of the various ingredients is usually 

 carried on in a mixing machine known as the Broughton mixer 



[pi. 24]. 



Anhydrous plasters 



This class of plasters has as a basis the dehydrated product 

 which results from calcination of gypsum at a higher temperature 



than is used in plaster of paris manufacture. Such plasters are 



• — . 



1 The set of plaster is determined in the same way as in the case of cements. 

 The apparatus commonly used is tlie Gilmore needle. A sample pat hav- 

 ing been made from the plaster, a needle of ^ inch cross section loaded 

 with a 4-ounce weight is placed on it. The initial set is completed as soon 

 as the needle fails to make an impression. 



