BLOOM
IT is not the intention of the author to go into any dissertation of the cause of bloom; for every varnish blooms more or less. In fact, all polished surfaces, whether they are varnished or not, show condensation, and surface deposits which are the equivalent of bloom.
Take for example, a mirror in your home; or a window glass; or a varnished piece of furniture. Unless these are continually wiped clean they will show a surface deposit, which is one type of bloom.
In paintings, bloom is the result of a variety of causes. Sometimes it is due to moisture which deposits. At other times it is due to the action of sulphur gases on the chemical compounds in varnish; and one of the most general causes is a surface deposit to which dust and foreign matter adheres. Some varnishes, like Mastic, bloom notoriously; and, if Mastic is to be used, it always should be mixed with ten per cent. of Spike Oil and five per cent. of heavy-bodied Linseed Oil. This, in a large measure, prevents the flatting and blooming of Mastic Varnish.
Damar Varnish, at times, shows the same defect, but not to such a great extent as Mastic, and Sandarac, the latter being an alcohol soluble varnish, which shows it least of all, has the defect of cracking very readily, particularly a year after it is applied.
Bloom can be removed in many ways: First, by gently rubbing with a silk handkerchief, which removes the surface deposit and polishes the underlying film of varnish; second, by taking heavy bodied Linseed Oil, diluting with half Turpentine, and applying that with gentle rubbing, which at once polishes the surface and removes superficial adherent bodies; third, by the use of any good, thin machine oil, such as is sold in this country under the name of “3 In 1.” A few drops of this may be applied on a linen handkerchief and gently rubbed, but then it must be wiped completely clean, because it is a non-drying oil, and dust and dirt stick to it with greater ease than they do to a drying oil.
Bloom also occurs in damp atmospheres, even on a painting which is not varnished. This is due to a certain physical—chemical cause, in which even dried Linseed Oil will absorb a certain amount of moisture. To overcome this, the painting should be placed in the warm sun, wiped clean, and then revarnished with a very thin varnish of either Mastic or Damar.