94 FLORIDA GEOLOGICAL SURVEY-I 5 TH 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 vitrifi¬ 
cation 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 tempera¬ 
ture 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 vitrifica¬ 
tion 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 1 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 out¬ 
side of the commercially verifiable class than fall in. it. . . . The 
preliminary stages of burning are vitally important in preparing the min¬ 
erals to combine and fuse into a solid solution. By the time the tempera¬ 
ture reaches 900° C. the compounds, (silicates, both hydrous and anhy¬ 
drous, oxides, hydroxides, carbonates, sulphides, free carbon, hydrocar- 
iOrton, Edward Jr., The Legal Definition of Vitrification, Trans. American 
Ceramic Society, Vol. XVI, p. 497, 1914. 
