Sometimes the objects are placed directly into a mixture of varnish and petroleum, or they are impregnated with melted paraffin. The former is preferable as a means of impregnation if there are cracks or holes, for the superfluous solution readily drips from the wood when it is taken out, while paraffin sets too soon to drain out of the cracks, and thus imparts an unnatural white appearance to the wood. Owing to the large size of the vessels which would be otherwise required, paraffin is only useful for small or medium-sized objects, but when making use of varnish one end of a large object[ [168] may be placed in the mixture while the solution is repeatedly poured over the object. After two or three days the opposite end should be placed in the solution. By repeating this process every part of the object will soon be thoroughly impregnated.
Objects of still greater size, such as a Viking’s ship, can only be preserved by painting the surface. In such cases it is advisable to begin with dilute varnish so as to allow the impregnating solution to penetrate as deeply as possible into the material, instead of merely forming a skin.
A solution of waterglass has in one instance been used for the preservation of a large boat, but the result is not satisfactory.
Leiner’s Method[169]. The wooden articles are laid in glycerine mixed with a small percentage of carbolic acid. The length of time during which they remain in the glycerine depends upon their size. When taken out they are lightly wiped and preserved without further treatment. If a growth of mould should occur it may be washed off.
Objects thus treated retain their moist condition and should therefore be very carefully protected from dust.
Speerschneider’s Method[170] (cp. p. [91]). Small specimens are heated for two hours in a mixture of
8 parts of rape-seed oil,
1 part of bees’-wax,
1 part of pine resin, and
2 parts of benzene.
Larger objects require a proportionately longer heating, but the mixture must not be allowed to actually boil. The moisture rises as steam and causes the solution to bubble. The bubbling however continues after the moisture has been driven off; great care must therefore be taken that the heating is not so prolonged as to cause the object to shrink. The highly inflammable nature of the mixture renders great caution necessary, and should it ignite, a lid, which should always be in readiness, should be put on the vessel. After impregnation the objects are wrapped in blotting-paper and laid in ashes for four days to prevent the access of air. The aim is doubtless to insure thorough absorption of the superfluous liquid which remains upon the object, which exposure to air would prevent by causing the mixture to set too rapidly. The same mixture can be used repeatedly, but each time two-thirds of the original quantity of benzene must be added.
Herbst’s Method[171]. The moist objects are boiled in a saturated solution of alum for two hours (hot water dissolves about 31⁄2 times its weight of alum), but if they are of some thickness the time must be proportionately longer. They are then taken out, and when the alum in crystallizing has made them more or less firm, the crystals adhering to the surface are washed off with warm water.
When thoroughly dry the wood is brushed over with hot linseed oil, which operation is repeated until no more oil is absorbed. A final thin coating of varnish or shellac is then given. According to Steffensen, the method followed at Copenhagen is to lay the objects in warm thin size for a quarter of an hour after impregnation with alum. This alum-method is there used for objects of oak, although the “Merkbuch” (p. 60) states that only the varnish-petroleum mixture should be used for impregnating this class of object.