In this process, the clay is taken almost as it comes from the pit, or, if it is too dry, a certain amount of water is added to it, not exceeding I0 to 1 5 per cent. Under these conditions blending becomes difficult, and requires powerful machines absorbing much motive force. Another grave disadvantage is the exfoliation of the paste as it passes through the die. With soft clay this is insignificant, but with hard paste it becomes very pronounced, and the slabs are formed of laminae placed one over the other like the leaves of a book. The slight exfoliation of soft or firm paste disappears in the press, thanks to the plasticity given to the clay by the water; with hard clay, considerable and repeated pressure is necessary to produce the same effect, and the action must also be completed by a strong firing, which will weld together all the parts of the tile.
If the pressure or the firing is insufficient, the laminated hollows in the clay will retain the water and the frost will chip the tiles, which in a very few years will be useless.
In this respect, then, manufacture with hard paste is more delicate than the previous methods, but by a judicious choice of clays, a suitable blending, and a strong firing, it gives good hard products, not very porous, and of clean-cut shape.
In spite of these qualities, however, the products have been much decried on account of certain accidents which we must explain. When tiles made of hard paste first made their appearance, their success, thanks to their beauty of shape and bright colour, was very great; and, as the moulding of them was somewhat cheaper than that of those made of soft clay, a large number of factories laid down plant for this method of manufacture. But, as often happens, too much advantage was soon taken of this economy; tiles were made without careful choice of clays, without proper preparation, and without sufficient firing, either to economise fuel, or because the clay used would not bear a great heat. The outer appearance did not suffer; on the contrary, the slight degree of baking of the products gave them a fine and uniform red colour, which made them extremely fashionable. Architects and engineers - and this is how they unconsciously helped to create a current of opinion which led to numerous disappointments - at last only used tiles of uniform tint and very red. But this tint is easily obtained by a slight firing, and therefore well-fired tiles, which are usually of a less agreeable and uniform colour, were partly neglected.
Time showed how wrong it was to forsake the wholesome traditions of good manufacture. When subjected to the weather the tiles at first behaved fairly well. But when their surface, which was not very pervious and strongly compressed, was attacked by the slow action of the water, they were rapidly destroyed, for the numerous hollows left by exfoliation filled with water, and the cold of successive winters caused these badly made tiles to fall into dust.
The reaction was all the more violent because lawsuits followed, and materials which had proved their utility for thirty or forty centuries were very nearly abandoned for roofing purposes! But the manufacturers with soft clay, between whom and those working with hard clay there was keen competition, skilfully took advantage of these circumstances and made hard-clay manufacture responsible for them; and in this they were not entirely wrong. In 1879 there appeared a pamphlet (La Tuile mecanique, by L. Laubtere, roofing contractor), in which, side by side with some excellent observations, other too extreme conclusions were stated. Acccording to the author, only soft-clay manufacture could give good tiles. Bad results may be obtained from soft clay just as much as from hard clay, and good tiles may be made with either. But it is certain that manufacture with hard clay is more difficult, more delicate, and requires good clays. Should we make a method responsible because some people apply it badly? Thus hard-clay products require a high degree of firing, and it is evident that if we use clays which cannot bear that firing, through being too fusible, or contracting unevenly and becoming warped, only a bad result can be obtained.
It is just the same if we work with soft paste made of poor clays, although the defects may be less pronounced. Soft paste, firm paste, and hard paste all may give good tiles, but clays cannot be used indiscriminately in any one of these processes, and it is prudent to diminish the disadvantages of hard paste by slightly increasing the quantity of water; it will then be in a better condition for working.
As for the consumer, his interests may be safeguarded, without inquiry as to the method by which the tiles are made, if he deals with solvent and well-known firms, and asks them for a warranty of ten years against rain and frost: this would never be refused by conscientious manufacturers who are sure of the quality of their goods.