Sand

To ensure good plastering good sand must be used, preferably Leighton Buzzard standard sand, which may be obtained from many pits in that neighbourhood. It should be clean, sharp, and hard. From practical experience it has been found that the size of the grains does not influence the strength of the mortar, although fine plaster requires fine sand.

As sand is cheaper than lime, care should be taken that too great a proportion is not used. The usual proportions are one part of lime to two parts of sand. On this point no hard and fast line can, however, be laid down, as the proportions vary with the fatness of the lime and other circumstances.

Sand is used in plastering -

(1) To produce regular shrinkage and prevent cracking.

(2) To enable the air to get into the plaster and crystallise it.

(3) To increase its bulk.

The different kinds of sand used are -

Of these pit sand is the best, next to it comes river sand, but sea sand should never be employed (unless it is impossible to obtain either of the other two), as the salts in it sooner or later discolour the wall paper, paint, etc., and render the wall damp, unless they be thoroughly worked out in running water - a tedious process.

Materials

The limes principally used for internal plastering where strength is not required are calcined carbonate of lime (known as fat lime, pure or rich lime) and, in London, chalk lime.

Hydraulic limes require only their own chemical composition of lime and silica to make them set, for they are able of themselves to set and harden under water. Natural hydraulic limes are obtained from Warwickshire, Leicestershire, and other places in the British Isles, whilst artificial hydraulic limes are made by mixing a sufficient quantity of clay with fat or chalk limes. The lime may either be mixed with clay and burnt raw, or may be burnt, slaked, ground, and afterwards mixed with clay and reburnt. The latter is the more usual method. Too much care cannot be taken in slaking lime, as, if this operation is not thoroughly performed, slaking will take place after use and form blisters in the work. \\ gallon of water will slake 1 bushel of lime.

Cements maybe divided into two classes - natural and artificial.

An example of the former is Roman cement, made by calcining the nodules found in the London clay and then grinding them to a fine powder; but it has now been almost entirely superseded by Portland cement.

Portland cement with a large proportion of sand is much used both in internal and external work, and also as a backing for the various white cements. It is unnecessary to slake Portland cement with water. In place of this it should be spread on a wood or concrete floor under cover for some weeks until thoroughly air slaked, to prevent the cement blowing in the work.

Cement mortar or plaster should be used as soon as it is mixed, and never after it has once set.

Plaster of Paris is known to the trade as Plaster, and is manufactured by gently calcining gypsum (which is largely found near Paris) after grinding. It is principally used for mixing with ordinary fat lime plaster to make it set quicker.

As plaster of Paris is extremely soluble in water, it should only be used in internal work, and great care should be exercised in keeping the air from it, as the moisture deteriorates it.

It is also largely used for cornices and cast ornaments, and for stopping holes in existing work, owing to its property of expansion and its quick-setting properties.

The proportion of ordinary stuff to plaster of Paris varies from 1 in 4 to 1 in 20, depending on the state of the weather and the speed with which the work has to be finished.

The following artificial cements have plaster of Paris as their base. They can be used where hardness, quick drying, and a fine or polished surface is required.

Parian Cement is composed of gypsum mixed with borax and cream of tartar and afterwards calcined. It is made in two qualities, coarse and superfine, and sets with a hard surface in from four to five hours.

Keene's Cement is made of gypsum, with the addition of alum (1 part to 12 of water at 950). After a thorough mixing it is baked to drive off the water. It is supplied in two qualities, coarse and superfine, sets very hard, and is used in positions subject to hard usage.

Martins Cement consists of gypsum and 1 lb. pearl ash dissolved in 1 gallon of water with a small proportion of sulphuric acid.

Sirapite is in composition similar to plaster of Paris, and is made from gypsum impregnated with petroleum, which is found in Sussex and Derbyshire.

Selenetic Cement is sulphate of lime (plaster of Paris) added to a lime possessing hydraulic properties, thus forming a rapidly setting cement which can be used with advantage as a backing for the white cements such as Parian.

Adamant has as its base gypsum and sand and fine sawdust, together with small proportions of dry glue, carbonate of soda, and borax. It can be readily applied, and dries quickly.

Robinson's Cement is composed of gypsum, with a mixture of tinal (an imported material) and alum. It is one of the best fire-resisting plasters, and is less costly than Keene's or Parian.

Mastic is used to point the joints between wood frames and the brickwork or stonework, and is heat-resisting, tenacious, and adhesive. It is made of stone powder, litharge, and linseed oil. A good mastic can also be made of two parts white lead, three of sand, and three of lime, thoroughly mixed with linseed oil.

Hair is mixed with the first coat of plaster on ceilings and partitions to bind the plaster together. It should be long, curled, strong, and clean cow hair, well separated before being mixed with the plaster; and care should be taken that it is not broken during the process.

Plasterer's Putty, or Putty, is made by simply slaking and straining lime through a sieve of 256 meshes to the square inch into a "putty bin," where it is left until it has attained the required consistency. In this state it will keep for a long time if kept moist.

Fine or Setting Stuff consists of slaked lime mixed with fine sand in the proportions of one part of putty to two parts of washed sand, used to allow the carbon dioxide to more readily enter the plaster and cause it to set.

Gauged Stuff is coarse putty or setting stuff mixed with plaster of Paris. This enables the plasterer to lay on several coats in internal work in rapid succession, as the addition of the plaster of Paris causes it to set very rapidly. The proportion of plaster of Paris used varies from 1 in 2 to 1 in 4.

Mixing And Application

Coarse Stuff is made by hand, and is a mixture of fine clean sand, chalk lime, and long sound hair.

The lime is first slaked in the following manner: - It is put in a large tub or vat and well stirred about until it slakes. When the slaking is completed it is then taken out of the tub and run through a sieve (of about 64 meshes to the square inch) in a creamy state into a wood bin, where it should be allowed to stand for at least a fortnight. When required for use the hair and sand are added, the former having been previously well beaten with a stick to separate it. The whole is then thoroughly mixed together with a long two-pronged fork called a "hair hook."

Coarse stuff is frequently made by grinding brick rubbish in a mortar mill, and afterwards adding lime and hair. The lime should be slaked and strained before being put into the mill, and the hair should on no account be added until the other contents of the pan are ground. The whole is then turned out as soon as the hair is thoroughly mixed. Care must be taken not to leave it in the pan too long, or the hair will be ground up, and then not perform its proper function. The proportions are -

One part lime, two or three parts sand, and one pound of hair to 3 cubic feet of coarse stuff.

Plastering is described as follows -

On Walls -