Acids
Acids may be divided into strong acids and weak acids. The former are highly dangerous if handled carelessly; the latter are not usually so, although some, like oxalic acid, are poisonous if taken internally.
The nature of an acid is not easy to describe simply. We are, most of us, acquainted with the strong acid sold commercially as spirits of salts, and referred to chemically as hydrochloric acid. We know this substance as a fuming liquid having a strong, pungent, smell which violently assaults the nostrils. We know that a drop of it on the skin will "burn" if it is not washed off promptly, and it is common knowledge that an alkaline substance, like soda, will neutralize the acid to the accompaniment of a good deal of perceptible activity. To see this effect drop a little spirits of salts on to a crystal of washing-soda (sodium carbonate). To explain adequately why this happens would be too great a digression. It is, therefore, better to take the common acids one by one, and make some remarks on the properties of each.
Acids and alkalis ought to be considered together. Caustic soda (sodium hydroxide), for example, is a strong alkali which will burn living tissue in much the same way as a strong acid. An acid atom is positively charged, whereas an atom of an alkali carries a negative charge. Positively charged particles are attracted by those with the negative sign, and the two combine more or less violently. The result of this combination is a neutral substance known as a salt. This is very much a simplification, but it will serve to explain the nature of the reaction.
An excellent example of this process may be seen in the combination of hydrochloric acid and caustic soda which yields common salt (sodium chloride). Again, sulphuric acid combined with caustic soda yields sodium sulphate, a substance used medicinally under the name of Glauber's salt. The resultant liquid needs to be evaporated over gentle heat for the salt to appear.
Acids are not used extensively in dealing with the restoration of antiques, but there arc some notable exceptions to the rule. Nitric acid is a strong, colourless, fuming acid, sometimes called aqua jortis. In contact with the skin, it inflicts dangerous burns, and any such accident should be immediately flooded with running water. This may be taken as the first-aid treatment for all such mishaps. Nitric acid has considerable uses in the manufacture of high-explosives. Mixed with three times its volume of hydrochloric acid, it becomes aqua regia. This liquid will dissolve such metals as gold and platinum, which are unaffected by other acids.
Hydrochloric acid is a strong acid sold commercially as spirits of salts. It has a number of uses for our purpose which will be further described in this volume.
Hydrofluoric acid is a colourless liquid giving off irritating and dangerous fumes. Unlike other acids it attacks silica and silicates, combining with the silicon to form the gas, silicon tetrafluoride. For this reason it is much used for etching on glass (which contains a large proportion of silica), in tests to discover the nature of certain kinds of old porcelain and glass, by fakers to remove slight decoration on old porcelain so that it may be redecorated with rarer, more valuable designs, and so forth. As it attacks glass, it cannot, of course, be stored in glass containers. This acid should never be disposed of by way of a glazed sink, and fumes from a loose stopper will damage glass and pottery glazes.
Sulphuric acid is a colourless, oily liquid with a thirst for water which makes it very destructive to all kinds of organic matter. Its common name is oil of vitriol, under which name it is sometimes used by jealous lovers to disfigure unfaithful paramours? something which it does very effectively as such injuries heal with large, contorted scars. In a dilute form it is used for accumulators. Sulphuric acid is very dangerous except in diluted form, and it has few uses for purposes described in this volume. Even as battery acid it needs to be treated with considerable respect, and splashes on the skin or clothes should immediately be washed off with copious amounts of running water. Great care should be exercised in diluting this acid with water. Acid should always be added to water, never water to acid.
Acetic acid was the first substance of its kind known to man. It is weak, and forms the acid constituent of vinegar. When this latter substance is mentioned in this volume as being needed for any purpose it is being used for its acetic acid content.
Commercial acetic acid is obtained from the distillation of wood. The salts are called acetates. It freezes at the comparatively high temperature of 15° C. As will be seen in the following pages, acetic acid has a number of uses in connection with our subject.
Oxalic acid is weak acid which is poisonous. It has its place on the shelf of the restorer's workroom as a 5% solution in water. In this form it is useful for removing ink-stains (q.v.).
Other less important acids mentioned hereafter are described under the appropriate headings.
Glass containers will not be affected by any of the acids mentioned, except hydrofluoric acid.
See Litmus Paper.