The temperature of the mortar and of the air in which the briquets are prepared is a matter of some moment. In 1877, Mr. Maclay 1 reported a series of experiments on Portland cements from which conclusions may be drawn concerning the effects of the temperature of the mortar. These experiments indicate that mortar having a temperature of 40° Fahr. when gaged, will attain greater strength in from seven days to three weeks than a mortar having an initial temperature of 70° Fahr. One is most likely to work somewhere between these two temperatures, but it may be mentioned that according to Mr. Maclay's experiments, it appears that mortars gaged at a temperature of 90° or 100° Fahr. also attain a higher strength than those gaged at 70° Fahr.

Similar experiments made by M. Candlot2 indicate that mortars gaged with cold water give but feeble resistance at first, but in from two weeks to one month, such mortars surpass in strength those gaged with warm water. M. P. Alexandre 3 immersed some briquets at a temperature of about 90° C. (194° Fahr.) for forty-eight hours and then at 15° to 18° C. (60° to 65° Fahr.) until broken, while other briquets were maintained at the latter temperature from the time of molding. The briquets that were broken at the age of four days showed that the highest strength had been obtained by the briquets which had been kept hot for forty-eight hours, but at twenty-eight days and three months those briquets which had not been subjected to this high temperature gave the highest strength.

170. Table 31 gives a few of the many experiments on this point made under the author's direction. It appears that the briquets made in a low temperature (34° to 37° Fahr.) are usually stronger than those made in the ordinary temperature of 65° to 68° Fahr. In some cases the difference was not very great, and in some of the tests the briquets made in the ordinary temperature gave higher results at one day and seven days than those made in the cold; but at twenty-eight days the cold-made briquets were nearly always in the lead, and in many cases this difference held good at three months and six months. Some of the results indicated that if the briquets were allowed to remain twenty-four hours or more in the cold air, it tended to counteract the beneficial effects of cold molding, but this point was not satisfactorily established.

1 "Notes and Experiments on the Use and Testing of Portland Cement," Trans. A. S. C. E., Vol. vi, p. 311.

2 "Ciments et Chaux Hydrauliques".

3 "Les Mortiers Hydrauliques".

Table 31. Temperature Of Materials And Of Air Where Made. Effect On Tensile Strength, Natural And Portland Cements

Notes: — All briquets made by same molder; each result is mean of five to ten specimens.

Results in columns headed "warm," temperature of materials used and air where made = 65° to 67° Fahr. Results in columns headed "cold," temperature of materials used and air where made = 34° to 37° Fahr.

a. In damp closet 36 hours, except 1 day specimens which were 12 hours in air where made and 12 hours in damp closet.

b. In damp closet 24 hours, except 1 day specimens which were 24 hours in air where made.

c. In damp closet 19 to 21 hours, except 1 day specimens which were 3 to 5 hours in air where made, 0 to 2 1/2 hours in damp closet and 16 1/2 to 21 hours in tank.

171. From the foregoing the following conclusions may be drawn: To make briquets of cold materials and allow them to remain some hours in cold air, retards the hardening of the briquets; but when briquets so treated are, after a few hours, placed in a medium of ordinary temperature, they gain strength more rapidly than briquets made of warm materials and kept continuously at the ordinary temperature of 60° to 70° Fahr. After being placed in a warmer medium, the briquets made with cold materials in cold air frequently gain strength at such a rate as to surpass in strength the warm-made briquets at seven days; the former almost invariably surpass the latter at twenty-eight days. In some cases it appears that this superiority of cold-made briquets is maintained up to six months, but in other cases the difference seems to disappear after three months.

Although these variations in temperature have not as marked an effect on tensile strength as have many other variations in manipulation, yet in carefully conducted experiments one should always operate in a constant temperature. As a matter of convenience, 65° to 70° Fahr. will commend itself, and this temperature may well be taken.