Alabaster is a soft, semi-translucent white sulphate or carbonate of lime; sometimes it has veins of yellow, red, or brown. A common" material generally known as alabaster is made of gypsum (plaster-of-Paris) by a special process, and is hardened by subjection to a heat of about 300" or 350 F., for from 12 to 24 hours. When almost cold it is immersed in pure water or in a weak solution of alum for a few minutes. These operations have often to be repeated. Sometimes the imitation alabaster is suspended in an alum bath until the alum crystallises on the surface. The material is then polished 'with a wet cloth. The real alabaster is worked in much the same way as is marble. It is easily turned in the lathe, strong chisels of the kind used by carpenters being employed for the straight work, and point tools for roughing out. For turning hollows the chisels are ground round. The cutting angles require to be more obtuse than for cutting wood. Alabaster is also easily worked in the lathe with tools such as are used in ivory and brass turning. It is a common practice to construct alabaster ornaments in two or more pieces and then to cement these together.
The following cements are recommended for the purpose.
(1) Mix the curd, formed by adding Apt. of vinegar to 1/3pt. skimmed milk, with the whites of five eggs. Well beat together and sift in sufficient powdered quicklime to form a paste.
(2) Mix together fay the aid of heat equal parts of plaster-of-Paris, yellow resin, and beeswax.
(3) Sift powdered quicklime into thin rice paste.
(4) Melt 2 parts of yellow resin and stir in I part of plaster-of-Paris. Apply hot to the warmed alabaster.
(5) Plaster-of-Paris mixed merely with water is a simple cement. Powdered sulphur may be added to this.
A means of decorating imitation alabaster is by etching. This process is executed by covering the surface, excepting those portions to be etched, with a solution of 1 part wax in 4 of turpentine thickened with a little finely powdered white-lead. The alabaster is then immersed in water for from 20 to 50 hours, according to the effect desired. The wax is then washed off with turpentine and the etched parts brushed with plaster-of-Paris. The real alabaster is etched in a similar manner, very dilute acetic or hydrochloric acid taking the place of the water. Another means of decorating alabaster is to colour it, but this is adopted as a rule only with the imitation material. Pigments that are not decomposed by contact with sulphate or carbonate of lime are added to the gypsum whilst in the wet state. Busts, medallions, etc., are coloured with sienna in powder or ground in water. For architectural purposes, the colour is added to clear size with which the plaster is worked up into the imitation material. Real alabaster may be coloured by applying hot liquid dyes or stains; the material itself should be sufficiently hot to cause the liquid to simmer.
For blue stain use tincture of litmus or an alkaline solution of indigo; for brown, use logwood extract; for crimson, use alkanet root dissolved in oil of turpentine; for gold, use a mixture of equal parts of white vitriol, sal-ammoniac, and verdigris; for green, use an alkaline solution of sap green; for red, use tincture of dragon's blood, alkanet root, or cochineal; and for yellow, a tincture of saffron. The rough alabaster is polished in the following manner. It is first rubbed with pumice powder or dried shave-grass (equisetum) and water, and afterwards with a paste of powdered and sifted slacked lime and water. The final lustre is given by friction with finely powdered talc or French chalk. Another method of polishing is first to smooth the surface with rifflers, scrapers, or glasspaper, and then to remove all tool marks with fine sandstone or gritstone, such as robinhood stone, water-of-Ayr stone, or snake stone. Then rub with pumice, either in lump or powder, and water, following with putty powder and water. Soap and water finish the polishing, or, instead of this, calcined tin may be applied with a linen muller in the form of a cushion.
Methods of cleaning alabaster and its imitation are the following.
(1) Immerse in milk of lime (slaked lime in water) tor some time, wash in water, and when dry dust with a little French chalk.
(2) Apply benzol or pure oil of turpentine.
(3) Wash with soap and water containing a little ammonia or soda.
(4) Rub with soap and wash in hot water. If stained, apply fuller's earth, pipeclay, whiting, or quicklime for three or four hours and then wash off.
(5) If very dirty, wash with dilute aquafortis or dilute muriatic acid.
(6) Mix pumice powder with verjuice and allow to stand untouched for two hours. Then nil) it into the alabaster with a sponge, and wash with fresh water applied with a linen cloth, afterwards drying with clean linen rags.