§ 100. Artificial Marbles have one incontestable advantage over the natural ones; that of composing surfaces of a great extent without seams or clasps, and of adhering so closely to the mason work that, when dry, they form part of the same body. Of all the methods of imitating Marble, there is none better than the composition of stuccos which has been long in use; these produce a beautiful effect in buildings. We shall explain the manner of their composition and application. We shall then speak of other artificial Marbles, of painting upon and coloring Marble, and shall conclude this part with interesting information relative to the manufacture of Venetian terraces.
§ 101. Stucco is a composition of a mixture of slaked lime, chalk, and pulverized white Marble tempered in water, designed to imitate different Marbles used in the interior of buildings or monuments.
There are different species of it, which may also have different bases.
Lime was formerly considered the only base of stucco. Our ideas have become greatly modified in this respect, but it is not yet proved that that which we make is superior to that of former times.
However this may be, as the hardness which the plaster may acquire is the most essential point in this art, it is to this that the workmen should first apply themselves.
This wholly depends upon the degree of calcination which is given to the plaster; and, as the stone which produces it is susceptible of some slight difference in its intrinsic quality, according to the locality in which it was found, one must study the degree of calcination necessary to give the plaster the greatest possible degree of hardness. The ideas which we give here relate only to the plaster of Paris.
Break the plaster stones in pieces somewhat larger than a large nut, put these in an oven heated as if for baking bread, and close the mouth of the oven. After some time has elapsed open the oven and take out one or two of the pieces, break these with a hammer, and if the calcination has penetrated to the centre of the piece so that a few bright points can be seen there, it denotes that it has arrived at its point of perfection; then draw out all the plaster as quickly as possible with a rake. If many of these brilliants are seen in the breakage, or none at all are visible, then it proves in the first case, that the stone is not sufficiently calcined, and, in the latter, that it is too much so.
Although the plaster becomes very hard when properly calcined, the surface is filled with a multitude of pores, and the grains are too easily detached from it to admit the polishing of it as of Marble. To remedy this objection, the plaster is diluted with water in which glue has been dissolved, which, filling the pores and fastening the grains together, permits the leveling and removing of the half of each grain, thus forming the polish.
Flanders glue is commonly used; some, however, mix with it isinglass, and even gum Arabic. Hot glue water is used for the solution of the plaster, as the want of solidity of the plaster, especially when it is not supported, demands that a certain thickness should be given to the works; to lessen the expense, the body or core of the work is made of common plaster, which is covered with the composition of which we have just spoken, giving it about an inch in thickness.
When the work is sufficiently dry, it is polished in nearly the same manner as the veritable Marble. A kind of stone is commonly used which is quite difficult to procure. This is a species of cos or coticular stone, which has finer grains than the sandstone, and which are not so easily detached from it. Pumice stone may also be used.
The work is rubbed by the stone in one hand, the other holding a sponge filled with water, with which the spot which has just been rubbed, is instantly cleansed, in order to remove what has been left on the surface of the work; the sponge should be frequently washed and kept always filled with fresh water. It is then rubbed with a linen cushion, with water, and chalk or tripoli stone. Coals of willow, finely pulverized and sifted, or even pieces of whole coals are substituted for this to penetrate better to the bottom of the moldings, water being always used with the sponge which absorbs it.
The work is finished by rubbing it with a piece of felt soaked with oil and finely powdered with tripoli stone, and afterwards with the felt moistened with the oil alone.
When a color is wished in the ground, it is only necessary to dilute it in the glue water, before making use of it to temper the plaster.
The stones for polishing can be adjusted to pieces of wood after the manner of jointers, or other joiner's tools; the surfaces of the work can thus be better prepared and the moldings more exact; it must not, however, be forgotten to always wash it in proportion to the rubbing.
When any particular Marble is to be imitated, dilute with warm glue water, in different small pots, the colors which are found in the Marble; with each of these colors temper a little plaster, then make of each a pebble nearly as large as the hand, place these pebbles alternately one above another, making those of the prevailing color more numerous, or thicker.
Turn these pebbles, thus arranged, upon the side, and cut them in slices in this direction, instantly spreading them upon the core of the work, or upon a flat surface.
By this means, the fantastical design of the various colors with which the Marble is penetrated, will be represented. If it is desired to imitate the Breccian Marbles, mix in the composition of these pebbles, when spread upon the core, pieces of plaster of different sizes tempered with the color of the Breccia, and those pieces, being flattened down, represent it very well. It should be remarked that in all these operations the glue water should be warm, without which the plaster will set too quickly, without giving time to work.
§ 102. When objects are to be presented on a colored ground-such as forests, landscapes, rocks, and vases of fruit and flowers-design them upon paper. Then puncture the outline of the designed figures, place them on the ground work when the polishing is nearly finished, and pounce them with a color differing from that of the ground; that is, of a black or red if the ground if the ground is white, and of white if the ground is black. Preserve the outline marked by the ounce by sinking them deeply with the point of a shoemaker's awl, after which, with several awls converted into small chisels by breaking off the points and sharpening them upon a grind-stone, remove all the ground which is included in the outlines of the ground which has been traced, thus forming cavities on the ground of about the third of an inch in depth.
§ 103. When all that is contained within the outlines has been thus removed, procure several little pots or cups holding sand from the fire or hot ashes, upon which pour glue water mixed with different colors; take a little of the plaster in the palm of the hand, color it more or less by carefully mixing it with the colored water, stir up the whole upon the palm of the hand with a painter's coloring knife, until the plaster begins to acquire some consistency, then take with the knife a quantity deemed sufficient, which place on one side of the interior of the cavity of the figure to be represented, pressing it with the knife, and smoothing the surface of the colored plaster which has just been placed, and which joins the outlines of the figure.
Then mix immediately in the hand another colored plaster, but of a lighter shade than the first, which place in the same cavity, by the side of the first; prepare four or five needles, by thrusting them parallely by the head into a small stick like the teeth of a comb, with which mix the latter color slightly with the first, so that the passage from one shade to the other may not be perceptible, and that the degradation may be visible.
Continue thus to place the brighter shades in the side of the light, until the excavation of the figure to be represented shall be wholly filled. Finally, flatten the whole lightly with the knife, and leave it to dry.
If, after the polishing, it is perceived that the shades are not distinct in any place, hatchings may be made in the plaster with an etching needle, and a darker colored and very liquid plaster inserted; these hatchings must be quite deep, that they may not be destroyed by the polishing which must afterwards be given to the whole work. This last method is used for cutting out the leaves of trees, plants, etc.
Undecided figures-as ruins, rocks, caverns, etc.-generally unite much better in this kind of painting than figures which demand exactness in the shading and correctness of design.
The paintings are polished in the same manner as the grounds, and if any little holes are discovered while polishing, fill them with plaster thinly diluted with glue water, and of the same color. It is even common, before using the oil for the polish, to pass a general tint of colored plaster and very clear water over the whole surface, in order to stop these small holes.
For all these operations the best and finest plaster should be chosen; that which is transparent seems to merit the preference.
For the colors, says the author of this process, all are suitable which are employed in fresco painting.
As it may seem strange that, in this manner of painting, we should have directed the use of the palm of the hand as a palette, we will give the reason for it. When a person dilutes the plaster with the colored water, he is obliged to use a certain quantity of water which would run off if placed upon a palette; by making a hollow in the palm of his hand, he retains it, and, by extending his fingers in proportion to the setting of the plaster, this singular palette, which was at first hollow, becomes flat when necessary. In addition to this, the heat of the hand prevents the plaster from setting too quickly.
Stucco is used either for ornaments, or for facings upon coats of impression: these last are composed, according to the methods described by Vitruvius, of several layers of mortar made with lime and possolaira. The stucco which was called by the ancients opus albarium, on account of its whiteness, or marmoratum, because it imitated Marble, which was also included in its composition, was placed upon the last layer of the coat; this is of a finer and thinner paste. It seems to have been made with a species of foliated gypsum, calcined and pulverized, which produced a beautiful plaster. When using it, the workmen mixed with it the same stone pulverized, but not calcined; in order, doubtless, to replace the marble dust, they compressed this last coating to give it more consistency and lustre. Some visible traces of pressure which have been found in several places, perfectly accord with what Vitruvius has said, and have even given reason for the conjecture that the instrument used in this operation, called by him baculi, was a kind of metal rule, light enough for the ends to support the prepared bands which serve as regulators to the workman.
§ 104. Such were the methods of our ancestors, and in practicing them they made in stucco the panels, columns, and pilasters which excite the admiration of men to whom ornament in architecture is a pleasure, and a means of satisfying a noble pride-since it cultivates a taste for the beautiful, and encourages the development of the arts.
It has long been observed that the palaces, chateaux, and houses in which stucco was employed for peristyles, stairways, and banquet halls, were always embellished with paintings, columns, and vases, either in concurrence with the stucco, or in the neighboring apartments. We have never seen anything more beautiful than the chateau de Mereville, on the beautiful estate of that name, in which M. de la Borde has collected all the choicest productions of the arts.
This hall was adorned with columns of white stucco, between which were panels of the same style, in which was hung four beautiful marine pieces of Vernet.
One can easily imagine that, entering in such a room as the peristyle of the apartments, they would penetrate into saloons of the greatest magnificence.
The use which has just been made of stuccos in the Palace of Foreign Affairs, on the banks of the Seine, also supports what we have just said. Thus stuccos are not enemies of Marble; on the contrary, they increase the demand for it.
It is not advisable then to neglect them; we should, on the contrary, encourage all essays tending to diminish their price, and to render them popular.
We will speak, then of processes indicated in 1836, by a Mr. Burrows, an Englishman, who imported among us methods of manufacturing stuccos and cements in hard stones.
"In the first place," says he, "for a plaster cement, I take a certain quantity of this material, which I reduce to a fine powder by the means ordinarily used for the manufacture of plaster of Paris; or else I take a certain quantity of casts of other articles which have been made of plaster of Paris, and reduce them to a fine dust by the action of fire, or by pulverizing them with a pestle.
"I then mix a solution of the following materials: Nine ounces of alkali (of the best American potash,) in six quarts of water; this solution should be neutralized with some acid, sulphuric acid is the best for this purpose. The solution must be stirred up, adding the acid gradually until the effervescence ceases; then add nine gallons and a half of water, making about eleven gallons of water in the whole. If any other alkali is employed, the quantity of water should be varied in proportion to this force; the water thus saturated, should be mixed with a sufficient quantity of the powder to acquire a consistency, or a condition suitable to be used or molded, whether in slabs, bricks, or in any other forms, which are then left to dry, and afterward subjected in reverbatories, ovens, brass crucibles of the kind used in gas works, or by some other means, to a sufficient degree of heat to bring them entirely to a red heat. If these articles are not heated entirely red, the parts which are not sufficiently calcined will be softer and less durable than the cement which has attained a proper degree of calcination.
"The quantity of the solution necessary for the mixture is about half of that of the prepared powder.
§ 105. "Secondly, for a limestone or chalk cement, I take a quantity of limestone or chalk, which I crush, and submit to the usual process for burning or calcinating lime. If I use the last process, I then reduce the lime to powder, either by exposing it to air, or by diluting it with water in the ordinary manner, (the dissolution by air is the best,) and treat it with a solution of alkali and sulphuric acid, as has been described for the mortar of Paris.
"But as less water is necessary for the mixture, the solution should be proportionably stronger. I dissolve nine ounces of alkali, of the best quality, in six quarts of water, to neutralize with the sulphuric acid in the manner explained for the Paris cement. I then add six and a half gallons of water, or perhaps a trifle more, and manipulate the solution to form slabs, which, when dry, I calcine in the manner described for the plaster cement. The solution necessary for the mixture is about a third part of that of the prepared lime. If the powder of plaster, chalk, or lime is used without being calcined, the calcination should be made in regard to the force of the liquid, and in proportion to the smallest quantity necessary to that object.
"The solution of alkali without the addition of the acid, can be employed for the composition of a cement of the powder of plaster, provided that it is afterwards calcined, but such a cement will not be as good as those made according to the methods which have been described.
"Cements of the powder of lime and chalk can also be made with a solution of acid; namely, two ounces, Troy weight, of sulphuric acid, dissolved in six gallons of water, and then calcined as before explained. But the cements made in this manner will not be as good as those made by the other methods indicated. The processes of incorporation and of calcination for these cements, and for the cement of plaster with alkali, are the same as those before explained."
After having described the methods or processes for the composition of hard cements by a mixture of alkali and suphuric acid with the powder of plaster, chalk or lime, and their subsequent calcination, by means of which the desired results with their advantageous qualities are obtained, Mr. Burrows adds: I shall now proceed to explain the manner of using them, and, as the cements made of the powder of calcined plaster, and those made of the powder of lime, possess different qualities, it will be necessary to establish a distinction between them.
§ 106. "I shall first speak of the cement made with the powder of plaster.
"The bricks or slabs before described, having been first reduced to powder and passed through a sieve in the manner used for hard cements, should be mixed with sand or gravel, and as the cement should produce no sensible heat while solidifying, it is desirable that the sand employed in the mixture should proceed from mineral or vegetable substances; for this reason well calcined or vitrified sands are the best for this use, and for any mortar or stucco that can be mixed or applied in the same manner as the cements of lime and other calcareous cements. As this plaster can be employed for coats of impression or other uses in which a slight absorption is desired, care should be taken not to put too much water in the mixture. It will generally be sufficient to put in a fifth or sixth part of the quantity of the materials. But it must be observed that, in this case, the success will still depend much upon the nature of those of which it is made. The same rule will serve for the application of cement without sand, whether employed alone, or as a light coating upon a stucco with sand.
"If a particular Marble is to be imitated, the paste should be applied to a very smooth surface. It is afterwards polished, and the colors can then be varied if they have not been amalgamated during the application."
§ 107. "I shall now describe the manner of using the cements made with the powder of lime and chalk.
"These cements, when fresh, produce much heat in solidifying. There is, therefore, less danger of their after deterioration when they are mixed with common sand, which should be vitrified, or calcined, as has been already said. The chalks having been pulverized, should be mixed with sand and employed in the same manner as the calcareous cements.
"From this, it is evident that the sands which have been described as being used in the application of my invention for stucco and other objects, can be mixed with the powdered plaster, lime, and chalk when the acids and alkalies are added, and that they are then subjected, with the other ingredients, to the action of heat or calcination, in which case it will not be necessary to add any sand when using them; and also that other alkalies or acids than those before mentioned can be employed, although, as none which I have experimented upon have succeeded as well, I give them the preference."
§ 108. Madame Bex of Paris, not finding the processes of which we have just spoken sufficient, claims to have discovered a method of less limited application, and which can particularly be extended to guard against dampness.
In pavements, flagging, and application upon the walls of ground floors, she has obtained, she says, the most successful results. This cement, which is as hard and compact as Marble, thus possesses those qualities of impermeability termed waterproof.
Stucco, on the contrary, being porous by the nature of its composition, is therefore easily accessible to dampness, which not only destroys its lustre but is a rapid cause of its deterioration. On this account it has not been employed for pavements and other uses before mentioned.
In order to obviate these inconveniences, Madame Bex has sought to ally stucco, as well as all other analogous compositions, and even soft and porous stone, with bitumens and natural vegetable, mineral and other bituminous mastics; that is, to line, in some sort, the stucco with these bitumens in such a manner as to thus preserve it from all humidity.
"When pavement or flooring is to be made, it is commenced by pouring the bitumen in molds of various shapes and sizes; before it cools, fragments of bricks, tiles, stones of all kinds, and even of wood, iron, brass, copper, lead, zinc, or any other material are thrown into it.
"Before placing the stucco upon the bitumen, what is technically called a gopte is made upon the materials which have been put in it; the stucco is then tempered upon a table, and glue water and the colors necessary to the Marble to be imitated, mixed in it.
"The stucco being thus compounded, a cake of it is made upon the table; this cake is cut into slices of four-fifths of an inch in thickness, more or less. These slices are forcibly pressed into the mold in such a manner as to unite with the substances incrusted in the bitumen as well as with the bitumen itself, after which the stucco is polished by passing sandstone over it with a martin, (a brass plate mounted upon stone.) The pores which are found in it are then stopped, and it is rubbed again with pumice to smooth it. Stones for polishing copper are used in the commencement of the polishing, then clear stones, and it is finally finished with touchstones.
"Floorings may be made of a single piece, in courses, or in squares as may be judged most proper.
"Applications of stuccos against walls are made by the same processes as those used for floorings or pavements.
"This useful application will give a considerable increase to the use of stucco."
§ 109. It is doubtful whether this process of Madam Bex will perfectly apply to the proper stucco. The omission of indicating the quantity and quality of bitumen and other ingredients which she employs, will probably somewhat hinder the use of it by workmen.
But the following recipe is simpler and easier, and is much used. Take the best plaster that can be procured, crush it, bake it as much as possible, crush, sift, and temper it in a solution of Flander's glue in water, let it dry, polish it with pumice and tripoli stone, and add a lustre with soapsuds and oil.
In the "Lime Burner's Manual," we have described several other processes for the composition of stuccos, analogous to those which we have just spoken.
Among other processes for coloring stuccos and giving them the appearance of Marbles, baked clay and metallic oxydes are used. These are the most solid. Oxyd (sic) of lead, ochre scorched and calcined, red ochre, and Roman vitriol baked in an oven, are also used.
One can also employ the oxyde or carbonate of copper, powdered green enamel, drops of forges, powdered scales of iron, etc. etc. The artist who is to make the imitation, must calculate the effects of the mixtures and the quantities to mix in order to produce the colors which he wishes to obtain, without losing sight of the fact that these substances produce very different results by the action of heat.
No book can point out the means of giving to the polishing of stuccos what they call a coup de main. This must depend on the practice and skill of the workman. The Marble worker who polishes well calcareous stones, will also polish stuccos well. The rules for success are the same in both operations.-Above all, it is important to do nothing roughly, and to always keep the work perfectly clean with a sponge dipped in clear water. The cushion which is used should contain tripoli and chalk finely sifted, and when the surface is perfectly smooth, the lustre is given by rubbing it lightly with a piece of grey felt, sprinkled with finely powdered tripoli, and then, in the last place, with another piece of felt moistened only with oil. We cannot recommend too much care that, when a polishing operation is to be performed, the piece to be polished should be perfectly freed from all the humidity contained in itself or acquired from the atmosphere.
§ 110. When the stucco was used among the ancients to form ornaments, it was worked, says Vitruvius, in two ways, either with the boasting tools, or in the mold. When they wished to make, we suppose, a bas relief or large ornaments, the workman designed upon the coat of impression, with the point of the boasting tool, the outlines of the figures which he wished to represent, and then modeled them in the paste of the stucco as our modelers do with clay.-The material dried too quickly in the hand of the workman to permit of any alterations. Thus a great facility of execution was necessary to succeed in this work, which fact renders the beautiful compositions of this kind found at Herculaneum and Pompeii still more worthy of admiration.
§ 111. The second method was employed for small successive works, such as ornaments for cornices, framings and ceilings. When the coat of impression was set in the necessary place, a mold was applied which left upon the fresh surface the desired imprint; the chipped edges were then adroitly raised, and the ornament remained simply and immovably fixed. The seams of the mold can be plainly observed in all parts; besides, it would be difficult to imagine that these different ornaments were first prepared, and that they applied them like the pieces of facings, or bas reliefs; the extreme tenuity of some of the details would render this impossible.
§ 112. When moldings are made in stucco, they can be jutted out with bores as in mason work, or if these bores are small, they may be fitted to a wooden jointing plane. To give a polish to all the indented angular parts, instead of a cushion of linen or felt, a piece of willow coal, or even of common coal is used, still moistening it with a sponge.
When a Marble veined with several colors is to be imitated, the different colors of the Marble are separately diluted in weak, warm glue, pastes are made of each of these shades; they are then flattened down and placed one above another, putting those of the prevailing color with the Marble to be represented, in the greatest number. All of these little cakes are then turned upon the side and cut in slices, which are immediately spread upon the trowel, care being taken to direct this trowel, and consequently the colored materials, in the same direction as that of the Marble to be imitated. When the Breccias are copied, pieces of soft Marbles, such as white and colored alabasters, are incrusted; these Marbles being rubbed down and their apparent surfaces polished, present, by reason of their forms, the usual pebbles of the Breccias.
In general, these coats of impression, which should be always at least one-fifth of an inch in thickness, demand much attention and particular pains; for instance, the colors for the surface should be properly arranged, the glue water should be always warm in order that the plaster may not set too quickly and that the rough cast above may be well prepared, etc.
If Etruscan, or other figures are to be made upon any ground, a pounce is applied when the ground is partly polished, then all the parts which are to receive the incrustations are removed with small chisels, gouges, and other tools suitable for this purpose, and cavities thus formed of from one-fifth to one-eighty of an inch in thickness, according to the outlines designated by the pounce. Small pots are prepared which are filled with the different colors necessary, and of which a paste is made with fine plaster in the palm of the hand, warm glue water being mixed with it; this paste is then introduced in the prepared cavities with a spatula or flexible knife and compactly pressed, the surface being smoothed down as much as possible. If there are several tints, or fillets of light and dark shades, the two edges are scooped out anew in the desired crockets, and the light tints of the reflex and the darker ones forming the shade projected are applied in the same manner. All the colors suited to this work can be found in commerce.
If, after the termination of the work, any parts of it have not the desired shade, those requiring alteration are pierced again with the etching needle or chisel, and paste inserted of the shade which is wished, care being taken to make these punctures so deep that they will not be rubbed down, and, consequently, effaced, by the polish and the lustre.
Some stucco workers put no plaster in their stuccos, but compound them simply of one part of quick-lime and two parts of pulverized Marble; others mix the quick-lime powder, and plaster together in equal quantities, and dilute the whole in a glue prepared as for painting upon Marble, but more transparent.
In general, stuccos should be executed in works which are not exposed to dampness, and upon very dry rough-casts of mortar, or plaster, otherwise this humidity repels them, and produces black spots upon the surface of the stucco, or the saltpetre which introduces itself cracks them and causes them to fall.