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Ice Colour
Ices are sometimes coloured simply with cochineal, but we append the following recipe, copied from good authority. Boil over a slow fire for five or six minutes in a pint of water, or clarified sugar if not to be used too quickly, equal quantities of cochineal (bruised) and salts of wormwood, an ounce of each. Add three ounces of cream of tartar, and an ounce of rock alum; but remove the saucepan from the fire before putting in the cream of tartar, etc., or it will boil over, and the whole mixture will be spoiled.
This is a dyer's formula written for the kitchen: a carmine lake made by striking cochineal with an alkali and then an acid in the presence of alum. The chemistry is sound, but the quantities belong to a dye vat rather than to anything meant to be eaten, and the finished colour cannot safely be used in ices.
- What the ingredients are"Salts of wormwood" is potassium carbonate, also sold as pearl ash; "rock alum" is potassium aluminium sulphate in lump form. Cream of tartar is potassium bitartrate, and "clarified sugar" means a strained sugar syrup, used here as a keeping medium.
- The alum is the problemAn ounce of potassium alum in a pint of liquid is roughly a 5 per cent solution. Aluminium salts at that concentration have no place in a food colouring; alum was included because it is a mordant that fixes the cochineal pigment into an insoluble lake, exactly as in textile dyeing, and there is no way to dose the resulting colour into an ice at a level that is both visible and within any modern limit.
- The alkali is causticAn ounce of potassium carbonate per pint gives a strongly alkaline, skin-irritant liquid that is boiled for five or six minutes. Nineteenth-century "salts of wormwood" was also variable in purity, and could carry other alkali residues from the ash it was leached from.
- Why it boils overThe warning in the original is real chemistry, not fussiness. Adding cream of tartar to a hot potassium carbonate solution neutralises it and releases carbon dioxide at once, foaming the pan up in seconds. That same acidification is what shifts the cochineal from a bluish crimson to a bright scarlet.
- UnitsThe pint here is the imperial pint, 20 fluid ounces or about 570 ml, and the ounces are avoirdupois weight, about 28 g each.
- What to use insteadCochineal itself is still a permitted colouring, sold as carmine or E120 and already prepared as a stable, correctly dosed liquid or powder. Buy it ready-made and follow a current recipe rather than compounding a lake at home.
Cochineal is the dried body of Dactylopius coccus, a scale insect farmed on prickly pear in Mexico and Central America and carried into Europe by Spanish traders in the sixteenth century, where it largely displaced the older kermes red. The technique in this entry is straight out of the dyehouse: alkali extracts the carminic acid, an acid then precipitates it, and alum fixes it as an insoluble "lake" pigment, which is why confectioners' colour recipes of this period read so much like dyers' recipes. Such recipes appeared in domestic manuals because a cook working before commercial food colours were reliably available had to raise her own tints, and cochineal was among the most dependable reds available. The alum and pearl ash would have been bought at the chemist's, and statutory standards of purity for goods sold for eating date only from the food adulteration legislation of the later nineteenth century.