102 FLORIDA GEOLOGICAL SURVEY— 15 TH ANNUAL REPORT 
of water absorbed expressed in percentage terms of the dry weight 
of the fired test piece and is obtained by the following formula: 
Absorbed weight—dry weight 
- X100=% absorption. 
dry weight 
A knowledge of the amount of water possible for a clay to absorb 
is very desirable as this property may seriously affect its usefulness. A 
small percentage of absorption indicates that the ware will absorb a 
correspondingly small amount of moisture. This in turn indicates a 
better resistance to frost action, less likelihood of the appearance of solu¬ 
ble salts and also indicates the general hardness of the ware as well 
as the general progress of vitrification. 
The porosity of a clay is an expression of the pore space it con¬ 
tains. The porosity is determined after each firing by boiling the test 
piece in water for one hour after it it has absorbed as much water as it 
will. This causes the piece to become saturated. Its weight suspended 
in water is also obtained. The porosity is expressed in percentage terms 
of the original weight of the test piece and obtained by the following 
formula: 
Saturated weight—dry weight 
---X100=% porosity. 
Saturated weight—suspended weight 
By use of the porosity determination the progress of vitrification is 
Known. The point of minimum porosity, or maximum density, is the 
point of complete vitrification. If plotted on cross-section paper where 
percentage of porosity and temperature are the factors, the resulting 
curve drops more or less uniformly until it approaches the temperature 
of vitrification, rising again as the temperature of viscosity is reached. 
When the porosity increases the clay is said to be overfired. The porosity 
of a clay product is regarded as an important factor in its durability and 
non-conductivity to heat. 
Porosity and absorption stand in direct relation to each other. The 
curves, when plotted, are essentially parallel. The determination of 
absorption is essential in structural materials and porosity in fireproofing 
ware. 
The transverse strength or tensile strength indicates the ability of a 
clay to withstand the shocks and strains of handling. The transverse 
