This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
"Coarse stuff" is a rough mortar containing 1-1 1/2 part sand to 1 of slaked lime by measure. This is thoroughly mixed with long, sound ox hair (free from grease or dirt, and well switched, or immersed in water to separate the hairs) in the proportion of 1 lb. hair to 2 cub. ft. of the stuff for the best work, and 1 to 3 for ordinary work. The sand is generally heaped round in a circular dish form; the lime, previously mixed with water to a creamy consistence, is poured into the middle. The hair is then added, and well worked in throughout the mass with a rake, and the mixture is left for several weeks to "cool," i. e. to become thoroughly slaked. If mixed in a mill, the hair should only be put in at the last moment, or it will get broken and torn into short pieces. If there is sufficient hair in coarse stuff for ceilings, it should, when taken up on a slate or trowel, hang down from the edges without dropping off. For walls, the hair may be rather less than in top stuff for ceilings. " Fine stuff" is pure lime slaked to paste with a small quantity of water, and afterwards diluted with water till it is of the consistence of cream.
It is then left to settle; the water rising to the top is allowed to run off, and that in the mass to evaporate until the whole has become thick enough for use. For some purposes a small quantity of hair is added. " Plasterers' putty " is pure lime dissolved in water, and then run through a fine sieve. It is very similar to fine stuff, but prepared in a more careful manner, and is always used without hair. " Gauged stuff" or "putty and plaster," contains 3/4 - 4/5 plasterers' putty, the remainder being plaster of Paris. The last-named ingredient causes the mixture to set very rapidly, and it must be mixed in small quantities, not more being prepared at a time than can be used in 1/2 hour. The proportion of plaster used depends upon the nature and position of the work, the time available for setting, the state of the weather, etc, more being required in proportion as the weather is damp. An excess of plaster causes the coating to crack. It is used for finishing walls and for cornices; in the latter, the putty and plaster should be in equal proportions.
Selenitic plaster is made with selenitized lime, otherwise known as selenitic cement, described on p. 585. The method of mixing the material for the first coat of plastering on brickwork is exactly similar to the process as carried out for mixing mortar. For plastering on lath work and other coats the following directions should be followed. To the same quantities of water and prepared lime, as given, add only 6-8 bush. clean sharp sand and 2 hods well-haired lime putty; the hair being previously well hooked into the lime putty. Lime putty should be run a short time before being used, to guard against blisters, which will sometimes occur. This mixture will be found to answer equally well for ceilings as for partitions. If the sand is very sharp, use only 6 bush. sand for covering the lath, and when sufficiently set, follow with 8 bush. sand for floating (or straightening). For common setting (or finishing coat of plastering), the ordinary practice of using chalk lime putty and washed sand is recommended.
But if a hard selenitic face is required, care must be taken that the prepared selenitic lime be first passed through a 24 by 24 mesh sieve, to avoid the possibility of blistering, and used in the following proportions : - 4 pails water, 2 bush. prepared selenitic lime (previously sifted through a 24 by 24 mesh sieve), 2 hods chalk lime putty, 3 bush. fine washed sand. This should be treated as trowelled stucco; first well hand-floating the surface, and then well trowelling. A very hard surface is then produced. For selenitic clay finish, take 5 pails water, 1 bush. prepared selenitic lime, 3 bush. prepared selenitic clay, 2 bush. fine washed sand, 1 hod chalk lime putty. This mixture, well hand-floated to a fair face, and then well trowelled, will produce a finished surface equal to Parian or Keene's cement, and will be found suitable for hospital walls, public schools, etc. Being non-absorbent, it is readily washed. The use of ground selenitic clay improves the mortar, and renders it more hydraulic. When the selenitic clay is used, 2 bush. may be added to 1 bush. prepared selenitic lime, the proportion of sand, ballast, etc, being the same as for prepared selenitic lime. The use of selenitic clay effects a considerable saving, as it is much cheaper than lime.
For outside plastering, use 6-8 bush. clean sand; and for finishing rough stucco face, 4-5 bush. fine washed sand, to the proportions of lime and water given.
"Rough cast" is composed of washed gravel mixed with hot hydraulic lime and water; it is applied in a semi-fluid state.
" Stucco " is a term very loosely applied to various substances which differ considerably from one another. These may be classed as follows: - (1) Compounds of hydraulic lime, formerly much used for external covering to walls.
(2) Mixtures of lime, plaster, and other materials for forming smooth surfaces on internal walls, chiefly those intended to be painted.
(3) All sorts of calcareous cements and plasters used for covering walls. "Common stucco" consists of 3 parts clean sharp sand to 1 of hydraulic lime. It was much used at one time as an external covering for outside walls, but has to a great extent been superseded by cements of recent introduction. "Trowelled stucco" is used for surfaces intended to be painted, and is composed of § fine stuff (without hair) and 1/3 very fine clean sand. "Bastard stucco" is of the same composition as trowelled stucco, with the addition of a little hair. " Rough stucco " contains a larger proportion of sand, which should, moreover, be of a coarser grit. The surface is roughened, to give it the appearance of stone.
"Scagliola" is a coating applied to walls, columns, etc, to imitate marble; it is made of plaster of Paris, mixed with various colouring matters dissolved in glue or isinglass; also with fragments of alabaster or coloured cement interspersed through the body of the plaster.
"Marezzo marble" is also a kind of plaster made to imitate marble. Upon a sheet of plate glass are placed threads of floss silk, which have been dipped into the veining colours previously mixed to a semi-fluid state with plaster of Paris. Upon the experience and skill of the workman in placing this coloured silk the success of the material produced depends. When the various tints and shades required have been put on the glass, the body colour of the marble to be imitated is put on by hand. At this stage the silk is withdrawn, and leaves behind sufficient of the colouring matter with which it was saturated to form the veinings and markings of the marble. Dry plaster of Paris is now sprinkled over to take up the excess of moisture, and to give the plaster the proper consistence. A canvas backing is applied to strengthen the thin coat of plaster, which is followed by cement to any desired thickness; the slab is then removed from the glass, and polished. Imitation marble of this description is employed for pilasters and other ornamental work. The basis of Marezzo marble, as well as of Scagliola, being plaster of Paris, neither of them is capable of bearing exposure to the weather.
The "artificial marble" now manufactured in London is made on the same principle as the Marezzo, but differs from it in the character of the cement used. A less expensive table is also substituted for the plate glass, and the canvas backing is altogether omitted.
Plasterers require a great variety of mouldings, ornaments, pateras, flowers, and other enrichments for the decoration of the work. These may be made either in plaster of Paris composition or in papier-mache'. Plaster ornaments are cast either in wax or in plaster, the latter process being used chiefly for large ornaments which have an undercut pattern. The ornament is in either case first modelled in clay and well oiled. In making wax moulds, the wax is melted, mixed with rosin, and poured in upon the model, arrangement having been made to prevent its escape; the whole is then steeped in water, and the wax becomes detached in one mass. When plaster is used as the material for the mould, it is laid on the model in plastic pieces fitted together, and then the whole, when dry, is immersed in boiled linseed oil. In casting, the plaster in a semi-fluid state is dabbed with a brush into the mould. Composition ornaments are made with a mixture of whiting, glue, water, oil, and rosin. The oil and rosin are melted together and added to the glue, which has been dissolved in water separately. This mixture is then poured upon pounded whiting, well mixed, and kneaded up with it to the consistency of dough.
When used, the material is warmed to make it soft, and is forced into boxwood moulds carved to the patterns required. Papier-mache' is a much lighter material for ornaments than either composition or plaster, and it is much used for the purpose. Cuttings of paper are boiled down and beaten into a paste, mixed with size, placed in a mould of metal or sulphur, and pressed by a counter-mould at the back, so as to be reduced to a thickness of about 1/4 in., the inner surface being parallel to the outer surface, and roughly formed to the same pattern. Papier-mache is sometimes made of sheets of paper glued together, and forced into a metal mould to give the pattern required. In some cases, a composition of paper pulp and rosin is first placed in the mould. This adheres to the paper ornaments moulded as above described, and takes the lines and arrises of the mould more sharply than the paper alone would do. Carton pierre is a species of papier-mache made with paper pulp, whiting, and size, pressed into plaster moulds. Fibrous plaster consists of a thin coating of plaster of Paris on a coarse canvas backing stretched on a light framework. This material has great advantages.
Large surfaces can be quickly covered without much preparation for fixing, as it is very light, and it can, if required, be painted at once.