136 ■ NEW YORK STATE MUSEUM 



resulting mixture when moistened gives better results in tensile 

 strength than pure Portland cement.^ This is due to the fact 

 that the particles of sand afford surfaces to which the cement 

 may adhere. 



The retarders used by most of the plaster manufacturers con- 

 sist of a mixture of various materials, and some of the mixtures 

 appear almost ridiculous. The essential constituents however are 

 certain forms of organic matter, among which are included glue, 

 glycerin, sawdust, sugar, oils, paper pulp, flour pack, fiber, oil 

 meal, molasses, Irish moss and tankage. The inorganic materials 

 consist largely of different salts of the alkalis and alkaline earths, 

 acids and other forms of cement. 



The ultimate strength of a retarded plaster will depend largely 

 on the changes that will take place in the material used to retard 

 the set. Many of the organic materials mentioned above would 

 undoubtedly decompose to a certain extent. If present only in 

 small quantities, their effect on the strength of the plaster would 

 probably be very slight, but the presence of any material which is 

 likely to decay is of no advantage to the plaster, not only from a 

 consideration of strength but also from a sanitary standpoint. 

 Great care should therefore be taken in the selection of an organic 

 retarder. 



The addition of a good mineral retarder, however, is likely to 

 prove an advantage, as the fact that the set is retarded gives 

 strength to the supposition that the resulting network of small 

 crystals will be much stronger because of the length of time re 

 quired for the set. It has been shown [p. 125] that, in the manu- 

 facture of Portland cement, the addition of gypsum in quantities 

 not over 2^ of the whole, increases not only the time required 

 for setting but the ultimate strength of the cement. 



It has also been shown^ that there is apparently a definite rela- 

 tion between the length of time required for setting and the 

 tensile strength, and up to a certain point those cements which 



^Newberry, S. P. Eng. News, Ap. 16, 1896. p. 252. 



^Ries & Eckel. Lime and Cement Industries of New York, pi. 91 to 

 98 inclusive. 



