A great variety of compositions are used by plasterers, some of which will be described.
Among the most important of these are cements of various kinds. Many of these are used also for building purposes, and have already been considered. Others are very deficient in strength and weathering properties, and are suitable only for covering the surfaces of internal walls. These will now be described.
The description of these was, to a slight extent, necessarily anticipated in Part II., but will here be repeated.
Cements, etc., used as Plasters. - Gypsum. - The basis of most plasters is a native hydrated sulphate of lime 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. White and light shades are the commonest in England, where it is found in Derbyshire, Nottinghamshire, Cheshire, and Westmoreland. It is also found in great abundance in the neighbourhood of Paris.
The very fine-grained pure white varieties are termed "alabaster," or, when transparent, "selenites."
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.
It is also added to various compositions in order to make them harden more rapidly.
Plaster of Paris is used for making ornaments for ceilings, etc., which are cast by forcing it, in a pasty state, into wax or gutta-percha moulds.
Where it is plentiful, as in the neighbourhood of Paris, it is used in all parts of house-construction where it will be free from exposure to the weather, for which exposure it is unfit, as it is very soluble in water.
There are three qualities of plaster of Paris in the market - the "superfine" "fine" and "coarse;" the two 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.
As before mentioned, the lighter varieties of Portland cement, weighing from 95 to 105 lbs. per bushel, are those 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, described at page 157, are used for external rendering, as mentioned at page 405, Part II.
Keene's Cement is a plaster produced by recalcining plaster of Paris after soaking it in a saturated solution of alum.
One pound of alum is dissolved in a gallon of water, and in this solution are soaked 84 lbs. calcined plaster of Paris in small lumps; these lumps are exposed eight days to the air, and then recalcined at a dull red heat.
The addition of half-a-pound of copperas gives the cement a cream colour, and is said to make it better capable of resisting the action of the weather.
Keene's cement is made in two qualities, the coarse and the 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 31/2 bushels, and the coarse in casks of the same size, and in sacks containing 3 bushels.
Parian Cement, sometimes called 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 two qualities of Parian cement in the market - the "superfine" and the "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 two latter cements are fatter, and produce sharper arrises and mouldings."l
As Keene's and Parian cement are not used for mortar, their tensile strength is of no practical importance. When allowed to set in air their strength was found by Mr. Grant to be as follows per sectional area of 21/4 inches : -
Martin's Cement is made in a similar way to Parian - carbonate of potash (pearl-ash) being used instead of borax, and hydrochloric acid being sometimes added.
It is made in three 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.2
2 Pap worth.
Robinson's Cement is made from alabaster (sulphate of lime) found in the Inglewood Forest near Carlisle. It has somewhat similar properties to Keene's and Parian cements, and can be used for similar purposes - for decorations, plastering, etc.
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."1
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 eight or ten 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 three parts of sand as a stucco. It is said to adhere well, to be hard when set and impervious to wet, and to be fit for mouldings or castings.2
The Keene's, Parian, and similar "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. This consideration, combined with their cost, and the moderate strength they attain even under favourable circumstances, makes them unsuitable for most engineering works.
In former years they were much used for covering external mouldings, etc. They were applied in a thin coat with great care, and looked well, but required painting periodically to compensate for the evaporation of the oil.
Several varieties were used on the Continent, but that best known in England was called Hamelin's Mastic.
This material, however, was expensive, and has been superseded by Portland cement.
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 be 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.