94 FLORIDA GEOLOGICAL SURVY-I5TH ANNUAL REPORT

tion and is accompanied by a sufficient softening of the mass to close all the pore spaces and render the mass impervious. The point of maximum shrinkage is also attained in this condition. The stage of viscous vitrification is characterized by further softening or swelling of the clay until it flows or becomes viscous.
 It is often quite difficult to recognize just when these three periods have been reached as the change from one to the other is frequently very gradual. In other cases the transition is quite sudden. The temperature necessary to change a clay from one of these conditions to the other is dependent upon the composition of the clay and is therefore variable. The difference in temperature between the points of incipient vitrification and viscosity may be less than 30* C. in calcareous ones to more than 275* C. in refractory clays.
 In the manufacture of clay products it is not possible to regulate the temperature of the kiln within narrow limits. It therefore becomes necessary to use a clay in which the points of incipient vitrification and viscosity are somewhat separated, particularly if a vitrified ware is to be produced. If a clay with a short firing range, as the range in temperature between incipient vitrification and viscosity is termed, is used there is danger of either not reaching the point of complete vitrification or going too far beyond this point and melting the contents of the kiln.
 Orton' has very completely discussed the nature of the vitrification process and points out that "any clay which is greatly overloaded with quartz sand, or with large proportions of carbonate of lime, or almost any other common mineral . vitrifies with great difficulty, and with a very imperfect degree of vitrification at best. It is not only not at all uncommon in practical work to find clays which will work well for ordinary porous clay products such as building bricks, and still not vitrify to anything approximating industrial requirements, but it may fairly be said that there are more clays used industrially which fall outside of the commercially vitrifiable class than fall in it. . The preliminary stages of burning are vitally important in preparing the minerals to combine and fuse into a solid solution. By the time the temperature reaches 9000 C. the compounds, (silicates, both hydrous and anhydrous, oxides, hydroxides, carbonates, sulphides, free carbon, hydrocar1Orton, Edward Jr., The Legal Definition of Vitrification, Trans. American Ceramic Society, Vol. XVI, p. 497, 1914.