Acids are used in leather processing but can cause damage if not applied correctly. Common issues include acid burns on hides from direct contact with strong acids like sulfuric acid. This can lead to darker marks after chrome tanning. Acid hydrolysis can weaken pickled pelts if the salt and acid levels are unbalanced or storage conditions are improper. Untanned skins swelling in acid can also be disrupted if not enough salt is added. To prevent issues, acids must be thoroughly diluted and mixed before adding, and temperatures monitored to avoid heat damage below collagen's shrinkage points. Proper salt and acid levels in pickle liquors and use of fungicides can prevent acid hydrolysis and ensure pelts' long-term storage.

1. Burns and swelling
The problem
Acids are an essential ingredient in many parts of the leather making process. They are used as a
preservative for part processed skins, eg pickled pelts, and to create the correct pH for chemical
penetration and reaction to occur, eg penetration of chrome tanning agents.
However, most acids used in the leather industry are highly corrosive and can cause irreversible
damage to collagen if not applied correctly. Common examples of acid damage are superficial
acid 'burns' to the surface of hides and skins, deterioration of pickled pelts during storage (acid
hydrolysis) and acid swelling. The cause Acid burns Strong acids, sulfuric acid for example, are
extremely corrosive and any contact of the hide or skin with undiluted acid will result in damage
to the grain surface. In addition, strong acids generate considerable amounts of heat when they
are diluted resulting in heat damage to the collagen. Heat damage occurring under acid
conditions causes the collagen fibres to stick together. Thermal damage occurring under acidic
conditions may be caused by: * Insufficient agitation of the hides or skins during acid addition *
Insufficient cooling of the diluted acid prior to addition * Insufficient pre-dilution causing heat
generation as it is diluted in the process liquor This damage may not be readily apparent at the
time, but will manifest itself later on in processing. The damage caused by the acid releases more
sites on the collagen molecule for chromium fixation. Therefore, the acid burns often show up
after chrome tanning as darker blue run marks. When the leather is dry, the surface in the
damaged areas is hard and brittle and very often cracks appear. Damage caused by the addition
of undiluted or hot acid is often in a characteristic run mark which has sharply defined edges.
Acid hydrolysis Pelts are pickled in salt and acid for storage; the salt partially dehydrates the skin
and the acid reduced the pH to slow down the growth of micro-organisms. If the pickling process
is conducted properly and the storage is appropriate, the pelts will remain in good condition for
many months. However, if the balance of salt and acid is incorrect or the storage conditions not
good, the acid can cause hydrolysis of the collagen to occur, resulting in discolouration and
weakening of the pelts. Acid swelling When untanned skins are brought down to an acid pH, eg
for storage or prior to tanning, it causes osmotic uptake of water to occur. This is controlled by
the fellmonger or tanner by the addition of salt. However, if insufficient salt is added, the
structure of the collagen fibrils is completely and irreversibly disrupted. The more the collagen
swells, the more sensitive it becomes towards heat; acid swollen collagen at a pH1.5 heat
damage can shrink at temperatures below 40°C in the absence of salt, but the addition of salt
(sodium chloride) can raise the shrinkage temperature to almost 70°C1. Prevention * Minimise
the risk of acid burns by careful addition of acids * Ensure that acids are thoroughly diluted and
cooled prior to addition * Make sure that the vessel is rotated for at least ten minutes after the
addition to ensure thorough mixing * Check the temperature of any process liquors being added
to pickled hides or skins; the shrinkage temperature of collagen can be as low as 37°C at very
low pH in the absence of salt. Acid hydrolysis of pickled pelts can be prevented by proper
adjustment of the pickle liquor. Pelts destined for long term storage: a fresh pickle liquor should
contain between 1.5-2.0% (w/v) concentrated sulfuric acid and 12-15% sodium chloride. At the
end of pickling, it should contain around 0.7% acid and 9-10% sodium chloride. The use of
suitable fungicide is recommended for long-term storage or storage at elevated temperatures. A
detailed laboratory analysis of an aqueous extract from a satisfactorily pickled pelt destined for
long-term storage in a temperate climate should provide levels of salt and acid given in Table 1.