Materials

A great variety of compositions are used by plasterers, among the most important being cements of various kinds. Many of these are used also for building purposes; others are very deficient in strength and weathering properties, and are suitable only for covering the surfaces of internal walls. In addition there are several mixtures made up of lime, sand, and other materials, distinguished by various names, and also used for covering surfaces of walls. The basis of most plasters is a native hydrated lime sulphate occurring as a soft stone, usually of a more or less crystalline texture, and varying in colour from white through shades of brown and grey to black. The very fine-grained pure white varieties are termed "alabaster," or, when transparent, "selenites." The raw stone is prepared either by simple calcination, or by calcination and combination with various salts of the alkalies. Plaster of Paris is produced by the gentle calcination of gypsum to a point short of the expulsion of the whole of the moisture. The raw stone is sometimes ground in the first instance and calcined in iron vessels. Paste made from it sets in a few minutes, and attains its full strength in an hour or two.

At the time of setting it expands in volume, which makes it valuable for filing up holes and other defects in ordinary work. It is also added to various compositions in order to make them harden more rapidly; and is used for making ornaments for ceilings, etc, which are cast by forcing it, in a pasty state, into wax or guttapercha moulds. Where it is plentiful, it is used in all parts of house-construction where it will be free from exposure to the weather, for which it is unfit, as it is very soluble in water. There are 3 qualities in the market - "superfine," "fine," and "coarse "; the 2 former being whiter and smoother in grain than the last. The superfine is sold in casks, and the other qualities in casks or sacks. Both casks and sacks contain 2 cwt.

Portland cement is much used by plasterers for external rendering, the lighter varieties, weighing 95-105 lb. per bush., being best adapted for this purpose. They set more quickly, and thus save expense not only in their first cost, but also in the labour that is bestowed upon them by the plasterer. Roman cement, and others of the same class, are used for internal rendering. Keene's cement is a plaster produced by re-calcining plaster of Paris after soaking it in a saturated solution of alum: 1 lb. alum is dissolved in 1 gal. water, and in this solution are soaked 84 lb. calcined plaster of Paris in small lumps; these lumps are exposed 8 days to the air, and then recalcined at a dull red heat. The addition of 1/2 lb. copperas gives the cement a cream colour, and is said to make it better capable of resisting the action of the weather. This cement is harder than the other varieties made from plaster of Paris, and is consequently used for floors, skirtings, columns, pilasters, etc.; it is also frequently painted to imitate marble. It is made in 2 qualities, coarse and superfine: the former is white, and capable of receiving a high polish; the latter is not so white, or able to take so good a polish, but sets hard.

The superfine quality is sold in casks containing 3 1/2 bush., and the coarse in casks of the same size, and in sacks containing 3 bush.

Parian or Keating's cement is said to be produced by mixing calcined and powdered gypsum with a strong solution of borax, then recalcining, grinding, and mixing with a solution of alum. There are 2 qualities in the market - "superfine" and "coarse." They are sold in casks and sacks of the same sizes as those used for Keene's cement. Parian is said to work freer than either Keene's or Martin's cement, and is therefore preferable for large surfaces, which have to be hand-floated before trowelling; but the 2 latter cements are fatter, and produce sharper anises and mouldings. Martin's cement is made in a similar way to Parian - potash carbonate (pearlash) being used instead of borax, and hydrochloric acid being sometimes added. It is made in 3 different qualities - coarse, fine, and superfine - the coarser kinds being of a reddish-white colour, and the finer pure white. It is said to cover more surface in proportion to its bulk than any other similar material. Metallic cement has a metallic lustre, is suitable for outside work, and is intended to dispense with colouring or painting, but is not much used. One variety is made by mixing ground slag from copper-smelting works with ordinary cement stone. Portland cement stucco is a mixture of Portland cement and chalk.

It is of a good colour and close texture; weaker than Portland cement, but not so liable to crack. Lias cement is produced from Lias shales containing a large proportion of soluble silica. It resembles Lias lime in appearance, sets in 8 or 10 minutes, and is used for lining water-tanks, or other purposes for which a light quick-setting cement is required. John's stucco cement is used as a wash or paint, and when mixed with 3 parts of sand as a stucco. It is said to adhere well, to be hard when set, impervious to wet, and fit for mouldings or castings.

These so-called "cements" or plasters are largely used for the best class of internal plastering, and, as they set very quickly, they can be painted within a few hours, which is a great advantage. They are capable of receiving a very high polish, to obtain which the surface is rubbed down with gritstones of various degrees of coarseness; afterwards stopped or paid over with semi-liquid neat cement which fills up the pores; rubbed again with snake-stone, and finished with putty powder. The plasters should not be used in situations much exposed to the weather, on account of their solubility.

The materials used in ordinary plastering are laid on in successive coats, which differ from one another in composition. In all of them the lime used should bo most thoroughly slaked, or it will throw out blisters after being spread. For this reason the " stuff" is generally made long before it is required, and left for weeks to cool. Pure or fat limes are generally used for the sake of economy, and for safety. Hydraulic limes would require special attention to prevent them from blowing. Moreover, the surface of plaster made with fat lime is more absorbent, and less liable to encourage condensation, than that of plaster made with hydraulic lime. Salt water and sea-sand should not be used, as the salts they contain would cause permanent dampness and efflorescence. The hair used by the plasterer in order to make his "coarse stuff" hang together is obtained from the tanners' yard. It should be long, sound, free from grease and dirt, thoroughly separated, beaten up, or switched with a lath, so as to separate the hairs, and dried. It is classed according to quality as Nos. 1, 2, and 3, the last being the best. A bushel weighs 14-15 lb.

White hair is selected for some work, but as it should all be thoroughly covered by the coats subsequent to that in which it occurs, its colour is not of importance.