762 NEW YOKK STATE MUSEUM 



object are trimmed off by means of a knife. Large objects such 

 as a statue or column have to be molded in several pieces, a sepa- 

 rate mold being required for each. Indeed extreme care has to 

 be exercised not to make single pieces v^hich are too large or too 

 complicated, otherwise they would warp and crack in drying and 

 burning. 



Drying of the wares needs to proceed with great slowness, and 

 in the case of larger pieces has even to be retarded by keeping 

 them covered with a damp cloth. The drying process is carried 

 on in warm rooms; where, in some terra cotta factories, coils of 

 steam pipe are laid under the floor. The shrinkage of terra cotta 

 in burning and drying is commonly about xs"- 



Much terra cotta is covered either with a soft dull enamel, or 

 glaze. This is commonly applied by dipping the green ware into 

 the glazing liquid, or it is put on by spraying, (pi. YY.) 



In the burning of the ware, simple forms can be piled on one 

 another in the kiln, but larger and more complicated pieces have 

 to be set in between slabs of firebrick, to shield them from any 

 pressure during the burning. Both coal and oil are used as fuel, 

 the latter having met with success at the worlcs of the !N^ew York 

 architectural terra cotta co. 



The color of terra cotta is either that of the body or is imparted 

 by a thin coating of slip. The slipping of terra cotta is extensively 

 practised, the advantages being that it makes the color of the clay 

 when burned immaterial, since the color of the object is given by 

 the slip coating. 



According to the composition of the slip, the surface is dull, 

 enameled or glazed. The composition of the coating must be such 

 of course that the coefficient of expansion of the body and of the 

 coating is the same, otherwise a crazing of the surface is sure to 

 ensue. 



The temperature reached in the burning of terra cotta depends 

 on the refractoriness of the clay. For calcareous clays the tem- 

 perature seldom exceeds 2000° F., but when semi-fire clays are 



