This substance, which is of great importance in the arts, more especially in printing and finishing cotton and linen goods, is often adulterated, and in other respects may be oi such a quality as to disappoint the manufacturer. Some inquiries which we have recently received upon this subject will be concisely and fully answered in the following extract from O'Neill's " Dictionary of Dyeing and Calico Printing : " Starch is a widely-diffused vegetable product; it exists in a vast number of plants, fruits, and trees, and seems to be one of the fundamental bodies of organic life. Its composition is very similar to that f sugar, being a compound of carbon with hydrogen and oxygen, in the proportions requisite to form water. It is extensively used in printing and finishing, but does not in either case exercise any actions of a purely chemical nature ; as a thickening it is only a vehicle for conveying the color or the mordant to the fiber ; as a finish it is only to give stiffness or fulness to the cloth. But its actions in many cases involve the play of chemical affinities, and should be minutely known. Pure wheaten starch, when closely examined under the microscope, is found to be composed of v.ery small globules. In commerce it is found in a peculiar state of aggregation, incorrectly said to be crystall-ized|; the quality of the starch is often judged and determined by the appearance of these columnar masses called crystals. No other starch but that from wheat takes the same form in drying. It is not prudent, however, to depend too much upon ! this as a test, for I believe the crystalline character can be communicated to other starches, and that it is not an essential character of wheaten starch, but rather an accidental one, due to a partial decomposition and breaking up of some of l the globules, which communicate a gummy nature and adhe- , sive character to the remainder, or to a residue of unremoved [ glutinous matters. Starch does not dissolve at all in pure wa- ) ter when cold, it mixes up, but then settles down, leaving the , liquid clear ; it dissolves in hot water, swelling out to a great extent ; it begins to dissolve, or the particles to burst, at - about 150 F., but color cannot be well thickened at this heat, ) it must be boiled to get a good result. Starch boiled with : acids, or acid liquor, thickens at first but afterwards becomes - thin, owing to the destruction of the starch and its conver- - sion into sugar ; colors should not, therefore, as a general . rule, be boiled until they begin to grow thin again—although 3 in special cases this is prescribed, and is an advantage, but it 1 is usually unnecessary, and likely to injure the color. , "A good wheaten atarch is white and clear, has sweet tasta 187 on the tongue, or at least an absence of bad taste, and, before dissolving in the mouth, shows an adhesiveness to the tongue; when mixed with water it should give a white milky fluid, without any particles of dirt floating on the top, and should settle down quickly, forming a solid hard mass at the bottom of the fluid. As a trial for its thickening powers a quantity may be boiled with water in the usual manner ; two proportions should be taken, one thicker than is generally required, and another thinner—for instance, one trial at one pound to the gallon, and both boiled with the usual precautions. The manner in which it behaves on boiling, as well as its appearance when boiled should be observed. A good starch will thicken gradually and evenly throughout, not in lumps ; it will keep smooth all the time with only a moderate amount of stirring, and when boiled will be of a clear, transparent, gelatinous appearance—not milky and opaque, nor breaking off" short when lifted with a stick. At two pounds per gallon it ought to be pretty stiff while hot, to pour out slowly, and for the most part adhere to the sides of a gallon mug, when this is inverted for a short time ; at one pound per gallon 'it should flow smooth and oily, without appearance of water or breaks in it. When cold, the thick trial should be very stifl; and feel tough and solid in the hand ; the skin should be of a tough leathery nature, and no water should be floating about—it will not be so clear as when hot, but still should be partially transparent ; the thinner trial should be also of increased consistence, and not show any water; it; should be smooth and not containing lumps. There are besides these characters a great number of others, too minute to record, which are combined in forming the opinion as to the quality of a sample of starch. It is a practical question, and nothing but a number of trials, upon all kinds of starches, will enable srny one to form a correct opinion upon this matter. Starch is sometimes adulterate I with mineral substances, a,s gypsum, sulphate of baryta,, or mineral white, China clay, etc. The existence of these substances make a starch boil rough and opaque; they can be discovered by burning some of the starch in a proper manner—if much earthy matter be left as a residue, it will be a sign of adulteration. It is sometimes understood that starch for finishing contains mineral matters, and a proportionable reduction in price is made, but oftener there is only one party cognizant of it ; at any rate a starch containing added mineral matter ought not to be used in mixing colors, however good it may be as a finishing starch. Inferior qualities of starch, under the names of seconds, slimes, and hair powder starch, are extensively used in the trade, and may be economically and easily employed in numerous cases ; for it is not necessary, in making colors, that a starch as pure as is required for domestic purposes should be used ; what is required is a good sound article, free from adulteration, not injured by acids or fermentation, and, if otherwise good, it does not matter whether it be in powder or in crystal, perfect white or a little grayish. Starch is sometimes injured by some of the gluten oi che fl:our being left in it. Such a starch does not keep well, soon goes watery, or putrefies, emitting bad smells. By scattering a little of this kind of starch upon a red hot iron plate the gluten makes itself apparent, by giving off* a disagreeable animal smell, like burning woolen, or leather, or the hoofs of horses. This kind of starch has never a good color, and, if in crystals, has a flinty hardness. Good starch does not contain more than ten or fifteen per cent of water ; the latter is the largest quantity it should lose in drying, at moderate temperatures."
This article was originally published with the title "Starch and Its Adulterations"