Objects of pure lead and pure tin are rare. If much oxidized they should be washed with warm water, dried, and impregnated with a gum-dammar solution or with paraffin wax (pp. [70] and [91]). If in a good state of preservation they may be freed from any earthy or calcareous coating or from lead carbonate by the cautious use of very dilute nitric acid followed by steeping in water.
Ceresole[ [160] cleans oxidized leaden seals with 10% acetic acid, neutralises the acid with ammonia, and after five minutes in alcohol coats them thinly with wax. The seals are preserved between glass dishes (Petri dishes), the space between the dishes being filled with cement. I employ Krefting’s method for leaden medals, using either zinc and very dilute sulphuric acid, or zinc dust and caustic soda. Occasionally the zinc dust becomes firmly cemented by oxide to the surface of the lead, and, if this is the case, great care must be used in removing it. The washing process also requires care. A very efficacious method is to allow a stream of warm distilled water, from which the dissolved air has been driven off by boiling, to flow over the object in a porcelain dish. I now omit any impregnation with paraffin wax, and instead recommend removal of the water by alcohol, drying, and coating with zapon. To preserve the specimens after treatment, more especially from the injurious action of perspiration from the hands, they are placed between dishes of glass or of celluloid[161].
(l) Gold.
Objects of pure gold usually need only be cleaned with soap and water and a soft brush; lime may be removed by the application of a 2% solution of hydrochloric acid. A coating of silver chloride occurring on gold which contains a large percentage of silver may be removed by ammonia, or, in certain cases, by the alternate use of dilute hydrochloric acid and ammonia.
A layer of red ferric oxide (see p. [53]) is of frequent occurrence upon gold objects, and may be removed by warming the object in a stronger solution of hydrochloric acid, but soft brushes will often serve the same purpose. Pure gold being very soft, only the softest so-called “silver brushes” should be used, and all pressure or bending should be avoided. If friable the object should be carefully impregnated with a solution of gum-dammar (p. [70]).
(m) Glass and Enamel.
If covered with a film of dirt, or if when in a collection objects of glass or enamel undergo any alteration, they should be washed or steeped in lukewarm water. When dry they should be treated with pure olive oil or poppy-seed oil, which may be diluted with benzine. The oil helps to restore the lustre to the glass and to bring out the colour of the enamel. When thus treated the objects should be carefully protected from dust.
A decomposition of ancient glass when deposited in a museum has been hitherto only rarely observed, but allusion may be here made to the so-called ‘sweating’ of glass which is a question of considerable importance in Industrial-Art collections. In this case preservation is insured by washing with distilled water, drying, and coating with zapon. Further particulars may be obtained from the paper by Pazaurek[162].