MECHANICAL DRAWING SUPPLIES 231 



smooth. There is some oil in the sizing preparation, and 

 consequently before the cloth is used, whiting or chalk is 

 rubbed into it to absorb the oil. 



277. Tracing Paper. Tracing paper is made from tissue 

 paper of an even texture, and possesses long and strong 

 fibers. This tissue paper is treated with oil and solutions 

 of resins and varnishes. 



278. India Ink. India or Chinese ink is always used in 

 making the tracing of a mechanical drawing, because of its 

 permanence, its distinct blackness, and because it is water- 

 proof. Moreover, India ink, because of its heavy composi- 

 tion, is less liable to " spread" and cause a blot. It is a 

 mechanical mixture of pure, dense lampblack and a solution 

 of gelatin, gum, or agar-agar. (Agar-agar is a gelatinous 

 substance obtained from seaweed.) This mixture forms a 

 black paste which is dried and pressed into cakes. It 

 was formerly the custom to use it in this form, but at the 

 present time it is easily obtainable in the liquid form ready 

 for use. 



Should the draftsman, however, buy India ink in the cake 

 form, he can easily prepare it for use by shaving off a portion 

 of the cake into water and stirring the mixture thoroughly. 



279. The Blue-Print. When a mechanic in the shop 

 receives a working drawing, it is in the form of a blue-print, 

 a blue paper on which the lines of the drawing appear in 

 white. 



A specially prepared paper, known as blue-print paper, 

 is used for making blue-prints. This paper is prepared by 

 the application of a chemical solution of red prussiate of 



