810 NEW YOKK STATE MUSEUM 



state or after burning. In the burning tbe ingredients of this 

 mixture unite during fusion and cover the surface with an im- 

 pervious glassy coat. 



Pottery glazes are generally of two kinds, raw or fritted. The 

 former consists usually of some mixture of metallic oxids, which 

 is sprayed on the surface of the raw clay. In the case of the 

 latter, the ingredients of the glaze are first fused together, forming 

 what is known as a frit, the frit is then ground very fine, and 

 mixed with water, this mixture being applied to the surface of the 

 green ware. It is specially necessary to prepare such a frit when 

 the glaze contains any soluble salts, the object of the fritting being 

 to render these salts insoluble. The fritting is usually done in a 

 special furnace, which has the bottorn sloping toward one point, so 

 that the melted material can be run out into a tank of water, at the 

 proper time. Certain frits, either on account of the difiiculty 

 with which they flow on melting or owing to the corrosive action 

 they exert when fused, can not be melted in the furnace, but are 

 fused in a special crucible or sagger. 



The proper preparation of the glaze often requires much skill 

 and experience; for the production of a uniform coating of glaze 

 on the surface of the ware is influenced by many different things, 

 such as the degree of porosity of the ware when glazed, the cleanli- 

 ness of the surface to be coated, the consistenev of the 2;laze, etc. 



If the density of the body is too great, or there happens to be a 

 film of dust or fat on the surface, the glaze is apt to contract into 

 drops during the burning. If the glaze is too' refractor}^, or the 

 kiln fire is not hot enough, the glaze will not be homogeneous, but 

 show little dots and pin holes. A frequent fault is the tearing or 

 springing off of glaze, which is due to the glaze and the body 

 having a different coefiicient of expansion. If that of the glaze 

 is greater, the body is apt to tear, whereas, if the reverse is true, 

 the glaze spalls off. It may be said in general that with an 

 increase in the amount of fluxes the coefiicient of expansion of a 

 glaze increases, while it decreases with the amount of acids. The- 



