Potatoes, peeled and mashed, and kept for thirty-six hours in a mixture of eight parts of sulphuric acid to a hundred of water, and then dried and pressed, form a white, hard substance very much like ivory, or, as one may say, like white boxwood. Lehner expresses his doubt as to whether artificial meerschaum pipes were ever made of this substance, but I have seen them, and can testify that they looked like meerschaum, and certainly were much harder than bruyere, or briar-wood. Whether they will “colour” I cannot say.
The principle by which potatoes, paper, and many other substances can be hardened like parchment or horn is curious. Potatoes consist of about seventy per cent. water and twenty-five per cent. of starch, the remainder being salts and cellulose, which forms cells surrounded by the grains of starch. “When such a substance is for some time brought into contact with diluted sulphuric acid, that which results is simply a contraction of the cells” (i.e., a hardening), “or a kind of parchmenting.” Thus soft paper is converted into parchment.
It is evident that chemistry is as yet in its infancy as regards the conversion of cellulose by acid into hard substances. Since cotton, paper, and potatoes all produce by this process different substances, it is probable that hundreds of organic, or at least vegetable, substances will all yield new forms.
There is a marked difference between paste made of starch or flour, each having its peculiar merits. The former is principally prepared from potatoes. To prepare the cement we mix it with a very little water, stirring it very thoroughly till it assumes a bluish appearance. A little more hot water is then added, and the mass left till an opal-like tinge indicates that it has formed. To this then add hot water ad libitum. As it is almost colourless in very thin coats, it is largely used to glaze and give body or weight to, and often to simply falsify, woven fabrics, which by its aid seem heavier. To increase this weight white lead and other substances are used.
To make the best flour-paste, flour should be kneaded in a bag under water till all the starch is washed away. What remains is a substance closely allied to caseine, or the white of egg. Combined with lime it forms a hard cement. A very slight admixture of carbolic acid (also oil of cloves) will keep paste from souring or decay. This acid has the property of destroying the growth of the minute vegetation which constitutes fermentation, just as other strong scents or perfumes are supposed to disinfect rooms, &c.
A very great number of other ingredients, such as the oxides of lead or zinc, manganese, baryta, sulphur, sal ammoniac, flint-sand, clay, salt, ochre, varnish, galbanum, or frankincense, enter into certain recipes, but those already given may be regarded as constituting by far the principal portion of all cements in ordinary use.
MENDING BROKEN CHINA, PORCELAIN, CROCKERY, MAJOLICA, TERRA-COTTA, BRICK AND TILE WORK.
Fictile or Ceramic ware embraces, roughly speaking, all that is made of clay, or mineral bases or materials, and which is subsequently baked to give it hardness. The better the material and the more intense the heat, or the greater the number of bakings to which most kinds are subjected, the harder and more lasting will they be. The old china ware which preceded porcelain, a great many specimens of old Roman vessels, and, for a more modern example, old Italian majolica and Hungarian wine-pitchers, made all within a century, are as hard as stone. They chip a great deal before they break, just as agate might do.
Terra-cotta is simply earth or clay “baked.” In most of the examples known as terra-cotta, earth predominates. Pure fine clay well fired is superior to what is generally called terra-cotta. Neither can we really class with it articles made of superior Portland cement, of which, as I have said, I have seen many made at Budapest which were like the finest hard slate.
Many writers confuse majolica with faïence; others regard the latter as what we should call crockery, or such ware as ranges between glazed terra-cotta and porcelain.