Lime enters into the composition of many cements, the simplest being the mortar formed by its admixture with water. But the quality of this is very much determined by that of the lime. The chunam of India, which resembles white marble or a fine white stone, is made of sea-shells burned to lime. A wonderfully hard, fine, white cement used by the Romans for their best mosaic-work, and which set with great rapidity, was made of shell-lime with the white of eggs. I have found the same composition worthless when made with inferior stone-lime.
A good cheap cement for porcelain and glass is combined as follows:—
| Starch or wheat flour | 8 |
| Glue | 4 |
| Purified chalk | 12 |
| Turpentine | 4 |
| Spirits of wine | 24 |
| Water | 24 |
Pour a part of the spirits and water mixed on the flour and chalk, add the glue, boil it down till the latter dissolves, and stir the turpentine into the whole. This can be used to make artificial wood with shavings or sawdust.
A very good cement for porcelain, and one which is colourless, is made by cutting the finest clear gelatine into bits, and dissolving it in vinegar of 50°, stirring it in a porcelain vessel until well mixed. When cold it will harden, but softens under the influence of heat, when it may be applied to the broken edges of the porcelain, which are to be pressed together. It will be perfectly hard within twenty-four hours. It is to be observed that the art of keeping such joined pieces together is the most difficult problem in mending. This cement is widely applicable to many objects, and also admits of considerable modification and additions, like all cements. As it is colourless, it may be combined with ivory dust, or white powders of baryta, magnesia, whiting, &c., to form artificial ivory with glycerine. With sturgeon’s bladder it makes a still stronger cement.
Lehner observes that glue has the property, when combined with acid chrome salt (sauren chromsalzen), of losing its solubility when exposed to the light, so that it can be used as a cement for broken porcelain and glass. If the juncture is to be invisible, take the purest white gelatine; otherwise the cheaper gilder’s glue will answer. To prepare the chrome glue, dissolve the gelatine or the glue in boiling water, then add the solution of double chromic acid alkali, or the red chrome alkali of commerce, stir it well up, and put it into tin boxes.
The formula is:—
| Gelatine or gilders’ glue | 5-10 |
| Water | 90 |
| Red chrome alkali | 1-2 |
| Dissolved in water | 10 |
To use, warm the cement, apply it to the broken glass, which must then be exposed for several hours to the sunshine.
Cracked bottles are mended by a very ingenious process, described by Lehner. The bottle is corked, but not tightly, and then exposed to heat about 100° centigrade. Then the cork is driven in tightly, which causes an expansion of the cracks, which are at once filled by means of a finely pointed brush with the silicate. Removed to a cooler place the glass contracts on the as yet fluid silicate, and the fractures are mended.