CHAPTER V.

MANUFACTURE OF TILES

SELECTION OF MATERIAL.

FOR drain tile it is necessary to manufacture an article of genuine earthenware, of sufficient strength to bear transportation and easy management, as well as to resist the action of water for a considerable period of time. These conditions may be found in a kind of earth less porous, more impervious, and finer in grain than that of which we make common brick; it must be similar in every respect to the earth of which roof tiles are made. We may, therefore, adopt as a general principle that, earth fit to make tile, is equally suitable for drain pipes, and that its preparation must, in both cases, be similar. Nevertheless, it may be remarked that flat and concave tiles for roofs are almost always manufactured by hand, while drain pipes are made cheaper and faster by machines.

The mortar about to be used ought to possess a degree of ductility and firmness which is not required for roofing tiles, especially when they are made flat. Pipe tile ought to be manufactured not far from the place where they are to be employed, on account of the cost of transportation, so as to render drainage easy and cheap. The materials of the compound must then be such as to furnish, at any time, at any place, cheap, substantial pipes.

Like other kinds of earthenware, this requires an essential distinction between the materials to be used for the composition of the mortar, and the elements that will constitute the piece completed or baked.

In the composition of the mortar, some compound foreign bodies are mechanically but not chemically combined. These compound bodies are materials for fabrication, but can be separated by water. In the baked or burnt mortar new combinations have been formed, against which water is powerless, so far, at all events, as to reduce the finished mass to the primitive materials. These combinations are multiple silicates, that is to say, silicic acid, combined with several bases-generally aluminum or lime-both in large quantities; at other times, and in less proportions, magnesia, oxide of iron, potash, soda and oxide of manganese. Burning, or baking, is the only means we have to obtain those fixed combinations that are subject to the action of neither acid nor water, and that are the more unalterable in proportion as the silicates are more exactly formed with their constituent elements, without any foreign admixture.

The essential elements are silicic acid and aluminum; from these may be obtained an earthenware which is fire proof, that is to say, it will not melt in the strongest fires of either forge or blast furnace. Aluminum, may sometimes be replaced in part by magnesia. The proportions of these indispensable elements are as follows:

When magnesia is present, it is generally found to the amount of 1 to 5 per cent.; there might be found as much as 25 to 35 per cent.

The accessory substances are still more variable in their proportions than the above; they are

These accessory elements give fusibility to earthenware, and, therefore, allow its constituent substances to combine. in such a manner as to form a resisting body; and this is performed with a temperature lower in proportion as the accessory elements are more abundant. In some baked mortars, there is carbonic acid (0 to 16 per cent.), when lime is present in sensible proportions. Water is almost always totally driven out of the mortar by the heat; and is present only in the paste in preparation; but here it performs an important office, by assisting to mix together the various materials which will bring into the paste the elements that we have described; it serves also to give them the required softness, to endow them with a certain adhesive force, and to promote their plastic qualities.

We term plasticity that quality which some soft matters have of assuming, under the hand of artists and mechanics, the forms that they wish to reproduce. We term those long pastes which are possessed of this quality in the highest degree, and short pastes those which have it in a slight degree only.

Plasticity is not absolutely indispensable for the shaping of ceramic pastes; we can mold them, by pressing the materials which are in the very state of dust; but a plastic substance yields better to the easiest and most usual mode of giving shape, and it is, therefore, much more desirable. 1

While plasticity is a condition of the first importance, in order to facilitate the shaping of the mortar into the desired forms, it offers great inconvenience when brought to an excessive degree. A paste which is too plastic dries

A gentleman exhibited at the Ohio State Fair, at Dayton, in 1860, a tile machine, which made tile from hydraulic cement, without the aid of water. The cement, of course, possessed no plasticity, but the tile or pipes were made by enormous pressure. If the tile thus made were placed in drains the moisture of the ground would cause them to harden, so as to be serviceable for many years.

up with difficulty, and great unevenness; articles manufactured from it are very likely, in drying, to lose their proper shape; they are very apt to crack, both during the period of desiccation, and in the bake oven or kiln. Excessive plasticity may be modified by other materials, which are either natural or artificial.

Sand is the natural correcting or tempering material. All sands are composed of silicic acid, or silicum, and of some foreign substances, from one to 9 per cent.; these foreign matters are aluminum, magnesia, lime, oxyd of iron, potash, etc.

The artificial tempering materials are: 1. Fragments of burnt brick or tile, reduced to powder. 2. Scoria, from the forges. 3. Sometimes sawdust.

As far as the drain pipe is concerned, it will not be necessary to discuss all the other materials which are used in the various productions of the ceramic art.

Any kind of sand may be employed for making drainage pipes, provided it be free from gravel, as it would interfere seriously with the molding.

As to plastic materials, although they may all be used, their qualities must be discriminated, in order to know how they shall be mixed together, and what proportion of tempering material, that is to say, sand, ought to be added to them. It may happen that some kind of earth may be found susceptible of being employed alone, and without any mixture. Let us see then what qualities each ought to possess:

1. The earth having received a sufficient quantity of water, must be malleable enough to assume all forms that may be wished; it must be firm enough to preserve those forms; it must be composed of particles sufficiently adherent, so that, when passing through the dye plate, this adherence is not impaired.

2. The earth ought not to contain any particle of pure chalk, even so small as the fiftieth part of an inch; baking it would produce lime; and lime, in contact with water, would slack and burst the pipe. There ought not to be any particle of either sulphuret of iron or of pyrites, as these would produce the same result.

3. It must dry readily, and with evenness.

4. The process of drying must be carried on, in such a manner as to evaporate the water which gave adherence to the particles, without producing cracks or deformities in the pipes.

We will now examine the various kinds of plastic materials which may be used in the fabrication of drainage pipes.

Natural plastic materials comprise clay, and clayey marl.

Clay is, in the potter's sense, an earth which forms a paste with water, working easily and hardening by fire.

Clay is plastic when it contains nothing but silicum and aluminum. This variety of clay which often bears the name of potter's clay, on account of its tenacity, does not readily admit water to penetrate, but when saturated it is very retentive of moisture.

Clay is fuliginous when it contains some lime, in the maximum proportion of 5 to 6 per cent., part of it as carbonate, and part may be in the state of silicate. This clay is still coherent, but less tenacious than the plastic above mentioned. It produces a slight effervescence with the acids, but this effervescence, caused by an emission of carbonic acid gas, soon ceases.

These two kinds of clay may be combined with an oxide of iron, and sometimes with particles of gypsum (sulphate of lime), or plaster.

Plastic clay, when not combined by these bodies, is al