224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



Attempts have been made to use a recording blank of conductive 

 material, or containing sufficient conductive material to allow of 

 omitting the subsequent graphiting or metallising of the blank; the 

 objection to this procedure has always been that such substances 

 offered too much resistance to the recording point. 



The commercial record is pressed in a substance the essential quali- 

 ties of which are that it should be hard at normal temperature, but 

 capable of being softened and made plastic by heat. It must be tough 

 and elastic enough not to be easily broken when pressed into disks of 

 about 2^ mm. in thickness ; it must be thoroughly homogeneous ; and 

 it must not be gritty in composition, as otherwise it will augment the 

 scratch of the needle, and wear off the point. Finally, the record 

 must be so hard, when cold, that it will retain the contour of the 

 sound groove, even after it has been played a large number of times. 

 Various substances and compounds have been used or suggested for 

 making records; celluloid, glass, papier-mache, vulcanized rubber, 

 casein, and shellac with an admixture of crocus powder. In nearly 

 all the compounds actually used shellac is the principal ingredient. 



The compound usually employed to-day is made up of shellac, wood 

 charcoal, heavy spar (barium sulphate) , and earthy coloring matter. 

 Various animal and vegetable fibrous materials, such, for instance, as 

 cotton flock, are added to give the record the required toughness. The 

 several ingredients are first finely ground and then carefully meas- 

 ured and mixed according to formula. The mixture is put into a 

 revolving drum, and the flock added. After being passed through a 

 magnetic separator to remove any metallic particles, it is next mixed 

 by heated rollers until a thoroughly homogeneous plastic mass is ob- 

 tained. The mass is now passed through calendar machines which 

 roll it out into thin sheets, and as it passes from the calendar it is 

 divided into sections, each section being about the requisite quantity 

 for one record. 



The records are pressed in hydraulic presses. The matrix is heated 

 and placed face upward in a mold on the lower half of the press, 

 being centered by a pin passing through the middle of it; the label 

 for designating the selection is placed face downward in the matrix, 

 and on this is placed, in a warm, plastic state, the quantity of material 

 required for one record. The press is operated, and the mass is imme- 

 diately distributed all over the mold. Both halves of the press are 

 furnished with cooling plates, through which a stream of water can 

 be passed so that the pressing surfaces can be immediately cooled, . 

 and the record mass consequently hardens quickly and retains the 

 impressions of the matrix. The record is removed, and its edges are 

 trimmed up with emery wheels; for the record material is too hard 

 to allow of any cutting instrument being used. The record is then 

 ready for sale. 



