The following may be regarded as the distinctive tests of dura bility: (1) absorptive power; (2) freezing test; (3) Brard's test; (4) acid test; (5) quenching test; (6) resistance to fire; (7) chemical analysis; and (8) microscopical examination.
Other things being equal, the less the absorption the more durable the stone. To determine the absorptive power, dry the specimen and weigh it carefully; then soak it in water for 24 hours, and weigh again. The increase in weight will be the amount of absorption. Table 5, page 21, shows the weight of water absorbed by the stone as compared with the weight of the dry stone—that is, if 300 units of dry stone weigh 301 units after immersion, the absorption is 1 in 300, and is recorded as 1-300.
Dr. Hiram A. Cutting, State Geologist of Vermont, determined the absorptive power * by placing the specimens in water under the receiver of an air-pump, and found the ratio of absorption a little larger than is given in Table 5. It is believed, however, that the results given in Table 5 * more nearly represent the conditions of actual practice. The values in the "Max." column are the means of two or three of the largest results, and those in the "Min." column of two or three of the smallest. The value in the last column is the mean for 20 or more specimens.
The probable effect of frost upon a stone may be determined by freezing and thawing a specimen several times while saturated with water, and then either measuring the amount dis integrated or finding the loss in strength. The loss in weight of even a poor building stone after any reasonable number of alternate freezings and thawings, say twenty-five, is too small to furnish any reliable estimate of the weathering quality of the stone. The deter mination of the loss of strength by freezing is unsatisfactory, since the difference before and after freezing is quite small, and since the comparison must be made between different specimens; and hence a reliable result would require a large number of tests, and even then the results would be only relative.
The test should be made with cubes of the same size, or at least with specimens having the same superficial area per unit of weight, as the disintegration is wholly on the surface. The cubes should first be brushed to remove any loose grains, and then weighed; they should next he immersed in water for 24 hours, and afterwards ex posed to alternate freezing and thawing for, say, twenty-five times; finally they should be thoroughly dried, brushed, and again weighed.
The loss in weight is supposed to measure the relative durability.
The first part of Table 6, page 23, shows the results of freezing and thawing each sample eight times.t The results are tabulated in the order of the losses in the freezing tests. The samples were tool dressed to rectangular faces; but were not of the same size, and hence the results are only approximate.
Brard's method of determining the effect of frost is much used; and although it does not exactly conform to the conditions met with in nature, it seems to be the best artificial means yet devised for determining the probable resistance of a stone to weathering. The test consists in weighing carefully some small pieces of the stone, which are then boiled in a solution of sulphate of soda, and afterwards hung up for a few hours in the open air. It is important that the solution be saturated only at or below 80° Fahr., as otherwise undue chemical action will he set up. The salt crystallizes in the pores of the stone, expands, and produces an effect somewhat similar to frost, as it causes small pieces to separate in the form of dust. The specimens are again weighed, and those which suffer the smallest loss of weight are the best. The test is often repeated several times. It will be seen that this method depends upon the assumption that the action of the salt in crystallizing is similar to that of water in freezing. This is not entirely correct, since it substitutes chemical and mechanical action for merely mechanical, to disintegrate the stone, thus giving the specimen a worse character than it really deserves. The following results were obtained by this method: * Table 6 shows the results of a series of tests to compare the losses by the artificial freezing test with those by the sulphate of soda (Brard's) test. The specimens were tool-dressed and had only ap proximately rectangular faces, but the pieces in the two tests were of nearly the same weight. The results of the two series do not agree very closely; but it is clear that the action of the sulphate of soda is much more powerful than that of freezing water.