Gum arabic dissolves in water, but not in alcohol, and therefore can not be employed for cements in the preparation of which solutions of resins in spirit of wine are to be used.
There are other products of vegetable life, which are also in commerce, called gums, but dissolve partly in spirit of wine. To this class belongs the gum ammoniac mentioned in some receipts for cements. As it is rather expensive, it is seldom used by itself as a cement.
Dextrine is extensively used in place of gum arabic in printing wall-papers, for stiffening and glazing cards and paper, for thickening the colors of calico printers, in making mucilages, etc. It is prepared by heating starch previously moistened with nitric acid in an oven, and can also be produced by heating paste with malt extract or very dilute sulphuric acid. There is a current anecdote which attributes the discovery of dextrine to a conflagration at a starch factory where one of the workmen who assisted in quenching the fire observed the gummy properties of the water which had been thrown over the torrefied starch.
Commercial dextrine forms pale-yellow to dark-brown masses. These masses dissolve readily in water, and form solutions which, as regards adhesive power, compare favorably with those prepared from gum arabic. The mucilage is prepared by simply stirring the pulverized dextrine with water to a thickly-fluid liquid.
To preserve mucilage unchanged for any length of time, and to prevent the disagreeable formation of mould upon its surface, it is recommended to dissolve some salicylic acid in the water to be used for preparing the mucilage.
Dextrine is usually prepared on a large scale by moistening 10 parts of starch with 3 parts of water acidulated with 1/100 part of nitric acid. The mixture is allowed to dry, and is then spread upon trays in layers about three-quarters of an inch deep in an oven, where it is heated for about one hour to 239° F. Sometimes large drums revolving over a fire are used, or, in order to keep up a uniform temperature, the starch is placed in a copper cylinder suspended in a vessel with oil which is heated to 356° F. The object of the addition of nitric acid is to allow the starch to be converted into dextrine at a temperature which would be inadequate to effect the transformation of starch alone.
Dextrine is also frequently prepared by allowing germinated barley or malt to act upon starch. Heat 350 to 400 parts of water to about 77° F., and after adding 5 to 10 parts of dry malt, raise the temperature to 140° F. Then add 100 parts of starch, and after mixing the whole thoroughly together, raise the temperature to about 158° F. for twenty minutes. The mass, which appears at first milky and sticky, will gradually become as liquid as water by the conversion of the starch into gum through the action of the malt. To prevent the conversion of the gum into sugar by the diastase of the malt, the fluid must be quickly brought to the boiling-point, and, after cooling, filtered and evaporated to the consistency of syrup. In cooling, the mass gelatinizes to a jelly, which after drying is hard and brittle.
According to Blumenthal’s method, a drum which can be hermetically closed, is filled two-thirds full with dry starch flour by means of a funnel. A stirring apparatus is then set in motion, and the acid which is contained in a graduated cylinder is sprayed into the drum by means of a special contrivance.
In a drum 5 feet long and 3¼ feet in diameter, 220 lbs. of potato starch can be uniformly mixed in 5 minutes with about 9 ozs. of nitric acid of 40° B., and the drum emptied by opening the slide. Starch thus treated may be brought into the oven without previous drying.
Heuzé gives the following method: Four and a half pounds of nitric acid of 1.4 specific gravity together with 300 quarts of water are mixed with 2,200 lbs. of starch, and boiled to form a mass which, when exposed to the air becomes dry. It is sometimes effected at 177° F., but it becomes a paste at 212° to 230° F. The starch changes into dextrine in an hour or an hour and a half at the most; it is white and soluble in water.