In wet-blue hides hexavalent chromium can form from unfixed trivalent chromium. Oxidation occurs more easily following aging of the leather, but the presence of some products that are used in retanning and certain pH values can favor its formation and release. Many widely used synthetic retanning agents are made through chemical reactions with formaldehyde so that a small amount in free form can be detected in the finished product. This percentage is not a real problem in the leather but sometimes it is better to use specific sequestrants of formaldehyde. There may be a case in which raw sheep and goats are worked, whose conservation process is unknown. In fact, formaldehyde preserves this type of material from bacterial action that damages its quality.