Tests of terra-cottas should always be made on marketable products.
If it is desired to know as precisely as possible the value of a manufactured article, it is better to work upon specimens at different stages of firing. It is often sufficient to examine the most slightly baked specimens, which will be easily recognised by their appearance, and especially by their being less hard and of slightly greater dimensions than the average.
The trade mark of the specimens shall be noted, their shape, the state of their edges and surfaces, also their colour.
In the case of bricks and quarries, their length, breadth, and thickness shall be measured. In the case of tiles, their length and breadth shall be measured, and sketches or sections shall be drawn which shall indicate, in a sufficiently clear manner, the hollows and projections as well as any fitting arrangements that the specimens may possess. Finally, for pipes, the internal diameter shall be measured, the effective length not counting the socket, the thickness of the walls, as well as the shape and arrangement of the socket, if any.
The dimensions of bricks and tiles should be verified, and should be identical for similar pieces; any differences exceeding i per cent, shall be noted.
When the laboratory has at its disposal a sufficient number of specimens from the same factory, it will be advisable to measure on the one hand those most baked or the smallest, and on the other hand those least baked or the largest. The observed differences, if any, shall be noted.
The observation of structure or homogeneousness shall consist of the examination of a fracture with the naked eye or a magnifying-glass.
There shall be noted -
(a) The appearance of the fracture, whether full - grained and with more or less pronounced toothings, or smooth or of conchoidal surface.
(b) The coarseness of the grain, stating, according to the classification adopted for natural building-stones, whether the grain is fine, medium, or coarse, and if it is uniform or of varied dimensions. (c) The homogeneousness, by observing whether the mass is entirely, partly, or slightly homogeneous, whether there exist more or less numerous and more or less accentuated planes of exfoliation or cleavage.
[Very fine grains(.2 mm. to.4mm.), fine grains(.3 mm. to .8 mm.), fairly fine grains (.5 mm. to 1.2 mm.), medium grains (1 mm. to 2.5 mm.), rather coarse grains (2 mm. to 4 mm.), coarse grains (3 mm. to 7 mm.), very coarse grains (5 mm. and above)].
1 These methods were adopted in 1895 by a Commission nominated by the French Government.
The determination of the specific gravity of the substance shall be made from the powder of pulverised fragments passed through a sieve of 900 meshes.
The powder shall be dried at a temperature of + 110°.
The specific gravity shall be determined by means of volumetres, by one of the methods now used, so as to obtain the first decimal figure with certainty and the second with an approximation of two units.
The liquid used shall be benzine or mineral essence.
The temperature should remain constant during the whole operation; it should not exceed +150.
The determination of apparent'density should be made, as much as possible, with specimens which have remained intact, after desiccation at a temperature of + 30° to + 40° C.
When the specimens have a regular geometric shape which allow of the volume being determined by measurement, a sliding-scale approximating to tenths of a millimetre shall be used; the weight shall be determined by means of a balance sensitive to half a centigramme.
When the specimens are of irregular shape, or present re-entrant angles, the volume, and thence the apparent density, shall be determined by finding the difference between its weight in air and in water. Care shall be taken beforehand to coat the surface with a varnish capable of preventing the entrance of water. A thin layer of melted tallow applied with a brush and spread with the finger fulfils this condition very well.
The absolute porosity shall be deduced from the difference between the specific gravity and the apparent density.
The determination of the weight of water absorbed, shall be made with a series of at least three specimens previously dried either in the open air or in a stove at a temperature of + 30° to +40° C. It is advisable as much as possible to work with whole specimens and not fragments.
After drying, the specimens shall be immersed in water to the depth of half their thickness for twelve hours, then completely submerged either for twelve hours, thirty-six hours, seven days, or twenty-eight days.1
The quantity of water absorbed, or the relative porosity, should always be calculated by volume, but the percentage of water by weight shall also be noted.