To receive stucco, the surface of the wall should be rough to form a key, thoroughly wetted, and freed from dust. The stucco is then laid on in a fluid state with a brush, like whitewash, after which a coat may be applied as in common rendering, unless the work is to be floated, in which case screeds must be formed round the margin of the wall, and vertically, at intervals of 3 or 4 feet throughout its length. These are filled out with stucco, which is smoothed by a straight-edge passed over it to remove any superfluous plaster, and then well rubbed with the hand-float and brought to a hard and glossy surface.
Plaster of Paris quickens the setting of the stucco, but should not be used for outside work, as it will not stand exposure.
Bough Stucco is an imitation of stone. It is spread in a thin coat on a floated ground, which should be in a half-dry state, and is then gone over with a hand-float covered with a piece of felt, which raises the grit of the sand and gives the surface of the work the appearance of stone.
This also has been superseded by cement, which is treated in the same way when it is required to have a rough surface.
Trowelled Stucco, consisting of 2/3 fine stuff and 1/3 sand, is worked upon the floated coat, which must be perfectly dry before it is applied. The stucco is beaten and tempered with water until it is of the consistency of thin paste. It is then spread with a large trowel over the surface, to the thickness of about 1/16 inch, as evenly as possible, and in small portions of about 2 or 3 square yards. The surface is then wetted and rubbed down with the hand-float (being sprinkled with a wet brush during the operation), until a surface is produced as hard and smooth as that of polished marble.
This process is used for surfaces to be painted, and for superior ceilings to be finished white. It is not so strong as the finishing with fine stuff.
Bastard Stucco is, like trowelled stucco, laid upon the second or "floated coat," but it is slightly different in composition, as it contains a little hair, is not hand-floated, and is finished with less labour than the other.
This and trowelled stucco are chiefly used for inside work intended to be painted.
Selenitic Plaster1 is made by adding a small proportion of plaster of Paris to hydraulic limes, which are then known as "prepared Selenitic Limes."
1 Selenitic material has been used in the new Imperial Institute.
The effect of this is to stop the slaking of the lime, and to convert it into a kind of cement.
1st, Pour into the pan of the edge-runner four full-sized pails of water.
"2d, Gradually add to the water in the pan 2 bushels of prepared selenitic lime, and grind to the consistency of creamy paste, and in no case should it be thinner.
"3d, Throw into the pan 10 or 12 bushels of clean sharp sand, burnt clay, ballast, or broken bricks, which must be well ground till thoroughly incorporated. If necessary, water can be added to this in grinding, which is preferable to adding an excess of water to the prepared lime before adding the sand.
1st, Pour into the tub 4 full-sized pails of water.
" 2d, Gradually add to the water in the tub 2 bushels of prepared selenitic lime, which must be kept well stirred until thoroughly mixed with the water to the consistency of creamy paste, and in no case should it be thinner.
"3d, Measure out 10 or 12 bushels of clean sharp sand or burnt clay ballast, and form a ring, into which pour the selenitic lime from the tub, adding water as necessary. This should be turned over two or three times, and well mixed with the larry or mortar hook.
"N.B. - Plastering on brick can be floated (or straightened) in one coat, and requires no hair.
To the same quantities of water and prepared lime, as given, add only 6 or 8 bushels of clean sharp sand and 2 hods of well-haired lime putty; the hair being previously well hooked into the lime putty. When the mill is used, the haired putty should only be ground sufficiently to ensure mixing. Longer grinding destroys the hair.
"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 bushels of sand for covering the lath, and when sufficiently set, follow with 8 bushels of 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 of water; 2 bushels of prepared selenitic lime (previously sifted through a 24 by 24 mesh sieve); 2 hods of chalk lime putty; 3 bushels of 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.
5 pails of water; 1 bushel of prepared selenitic lime; 3 bushels of prepared selenitic clay; 2 bushels of fine washed sand; 1 hod of 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 bushels may be added to 1 bushel of 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 or 8 bushels only of clean sand, and for finishing rough stucco face use 4 or 5 bushels only of fine washed sand, to the proportions of lime and water given.
"Plastering on Walls can be finished by the above processes, as two-coat work in 24 hours, while the ceilings can be floated immediately after the application of the first coat, and be set in 48 hours."
The advantages of this material for plastering are - its cheapness, as it can be used with a very large proportion of sand; its quick setting, which enables the coats to be applied rapidly in succession, and prevents delay.
Selenitic lime or mortar should not be used in conjunction with gauged stuff for cornices, screeds, etc. No more mortar should be gauged than can be used the same day.
In applying selenitic plaster to quartered partitions or ceilings, care must be taken that the supporting woodwork is thoroughly seasoned, for the plaster is rigid and will be cracked if the timber warps and twists.
Bough-Cast is used as a cheap protection for external walls. The surface of the wall is first "pricked up" with a layer of "coarse stuff," upon which a coat of similar composition is evenly spread; while this is wet, and as fast as it is done in small portions, rough-cast (p. 179), in a semi-fluid state, is thrown upon it with large trowels from buckets, forming a rough adhering crust, which is at once coloured with lime-wash and ochre.
Depeter consists of a pricked-up coat with small stones pressed in while it is soft, so as to produce a rough surface.
Depretor is the name for plaster finished so as to represent tooled stone.