1st. Water smoking, which lasts about 72 hours during which the temperature is raised gradually to 350 degrees Fahrenheit.
2nd. Heating, during which the temperature is raised to 800 degrees Fahrenheit in 24 hours.
3rd. Oxidation, during which the temperature is raised to 1,400 degrees Fahrenheit in 84 hours.
4th. Vitrification, in which the temperature is raised to 2,100 degrees Fahrenheit in 48 hours, and finally,
5th. Glazing, during which the temperature is unchanged but salt (NaCl) is thrown in and allowed to burn.
Oxidation must be complete before vitrification is started as otherwise blisters will be raised due to imprisoned carbon dioxide. The important points in vitrification are to make the required temperature within a reasonable time and to maintain a uniform distribution of heat throughout the kiln. When vitrification is complete as shown by a glassy fracture of a broken sample taken from the kiln, glazing is accomplished by throwing a shovelful of salt on the hottest part of the fire. About five to six applications of salt from two to three hours apart may be needed. The kiln is then allowed to cool and the manufacture of the pipe is complete. The completeness of vitrification is indicated by the amount of water that the finished pipe will absorb. Completely vitrified pipe will absorb no moisture. Soft-burned pipe may absorb as much as 15 per cent moisture.
Vitrified clay blocks are made of the same material and in the same manner as vitrified clay pipe.
The following data on vitrified pipe have been abstracted from the specifications for vitrified pipe adopted by the American Society for Testing Materials.
Pipes shall be subject to rejection on account of the following:
(a) Variation in any dimension exceeding the permissible variations given in Table 36.