There are two principal reasons for the development of a bloom on the surface of leather

• micro-biological agents, i.e. the growth of fungal or bacterial colonies and
• crystallisation of material emanating from within the leather.

Microbial Growth

Microbial growth this is almost always indicative of storage under inappropriate environmental conditions. Most types of leather are hygroscopic and will draw in moisture from a damp atmosphere until a point is reached where micro-organisms can thrive. The visible sign of this is usually a white, powdery deposit on the surface but there is often penetration into the substrate which may not be immediately apparent. The biological processes are fuelled by digestion of the substrate resulting in an irreversible breakdown. The important remedial actions are to halt further activity by returning the leather to a drier state and by the use of an appropriate biocide. The degree to which the original appearance can be restored is largely dependant on the nature and severity of the contamination. In many cases it will be possible to achieve a good result by brushing or lightly swabbing the surface, followed by an application of wax based penetrative leather dressing.

Crystalline Surface Deposits

Crystalline surface deposits known in the leather trade as "spues", arise because of the migration towards the surface of unbound, mobile components from within the leather. They are unsightly but are not, generally speaking, harmful and they often come about as a result of cyclical changes in environmental conditions, i.e. fluctuating temperature or humidity. Broadly, these deposits may be subdivided into salt spues and waxy spues. The classical method of differentiating is by applying a local source of heat, for example a match flame, which will usually cause a waxy spue to melt and disappear - at least temporarily, whereas a salt spue will be unaffected.

Salt deposits are often water soluble and can be readily swabbed away; however, as the leather dries out again further salt may be brought up to the surface causing a recurrence of the problem. It is important to bear in mind that soluble components are always carried towards the side where evaporation is taking place (this is the principle on which chromatography is based). Wet leather should, if possible, be dried with the grain side against an impervious sheet to encourage migration away from the grain and help prevent excessive build-up and crystallisation of salts on the grain surface.

Waxy deposits are, generally, not water soluble. Localised heating , as described earlier, or polishing may be all that is required to improve the appearance but neither of these methods actually removes the surface contaminant. It may be possible to remove this by swabbing with a weak solution of detergent or lactic acid but often it is necessary to employ an organic solvent. The same principles apply as described in the paragraph above, i.e. after washing off the excess, final drying should take place into the leather so as to carry any residual material away from the surface.

A special case exists for soap deposits which can form when certain metallic compounds react with oils in the leather. The types of soap most likely to cause a problem in leather are those resulting from a reaction between basic chromium or aluminium tanning agents and emulsifiable oils introduced during the leather-making process or naturally occurring free fatty acids. These soaps are highly insoluble and may be difficult to deal with. Oxalic acid has been suggested elsewhere as a suitable material for removing soap deposits but there are two significant disadvantages to its use - firstly, it is a moderately strong acid which could lead to damage if an excess were left in the leather, and secondly, it is poisonous by skin absorption and so should be handled with extreme caution. A better and safer option is to swab with iso-propyl alcohol which is a fairly good solvent for many of these soaps

Aluminium and chromium soaps are not likely to be of concern to users of regular vegetable tanned bookbinding leathers. Chromium tanned leathers can be very difficult to work with and chromium/vegetable combination tannages have been shown to have poor performance under accelerated ageing tests. Aluminium tawed and aluminium/vegetable tanned leathers, which perform very well in these test, are sold as specialist "conservation" grades, so there should be no confusion as to whether or not aluminium is present. Other metals which may give rise to soap formation include calcium and magnesium from hard water areas but the tanner should have taken appropriate steps to deal with any potential problem and, in any event, these will only be present in trace quantities.

It goes without saying that any methods which are to be applied to a leather surface should be tested out first on an inconspicuous area, for example, to check for any effect on the underlying colour. The finish film on certain types of leather may be softened and rendered much more susceptible to damage while wet, and care should be exercised in handling the leather. Safe working practices should be adhered to when using any chemicals, this includes the use of appropriate personal protective equipment and the provision of adequate ventilation when using volatile solvents.