Formaldehyde (a pungent smelling gas) is water soluble and its solution is known as formalin(toxic and may develop acidity). Presumably tanning was observed for the first time when meat together with the skin were smoked. Formaldehyde is probably the only tanning gas. Stabilized formalin(containing 8-10 % MeOH) contains 40 % formaldehyde and is used for tanning white, washable leathers with the grain split or shaved off.
The skins are prepared to a pH 4 or 5 and drummed in 3% formalin with the least possible amount of water. A temperature of 30 C is beneficial. After runtime of 4-5 hours, they are left in the closed drum overnight and then 'ashed up' (1-1.5 % soda ash, 50-100% water) until pH is not less than 8. At this pH formaldehyde rapidly fixes to the skins. At higher pHs over tannage of grain side (with no penetration) occurs. If production of grain leather is intended various(modified) alkali systems are recomended (use of Mg salts) to avoid this danger.
The amount of aldehyde being attached to the hide is small, from 0.2 to 2 %. Part of this may remain unbound. Aldehydes combine with the basic amino group of skin protein. In alkali some condensation(aldol) to larger molecules give fullness to the leather.
Aldehyde tanned leathers have reduced ability to fix acid (basic groups have reacted).Similarly they can reduce fixation of some vegetable tans and dyes. Aldehyde tannage reduces isoelectric point of hides, so that at any pH it has a lower cationic charge than raw skin, and mineral tanned leather. This can reduce fixation of anionic sulphated oils so that such fatliquors penetrate better, but may washout more easily.
Ts is raised only to 70 C. Leather becomes whiter as exposed to light and readily absorbs water.
RNH₃+ + CH₂O --> R-NH-CH₂OH + H+
RNH-CH2OH + NH₂CO-R -->  RNH-CH₂ - NH-CO-R + H₂O

Gluteraldehyde: (OCH-CH₂CH₂-CHO)
Under equivalent conditions it can give higher degree of tannage and increase of Ts than formaldehyde at lower pHs. Attention has been given to the phenomena that some degree of gluteraldehyde particularly on mineral tannages improves leathers resistance to perspiration.
Gluteraldehyde forms semiacetal bonds with hydroxyls of hydroxyproline, hydroxylysine and serine. With phenols it yields insoluble compounds, so can not be used with vegetable tannins.

With amino groups it my react in 3 ways:

Gluteraldehyde in 25-50% aqueous solution is found to oligomerize (3-5 molecules).This may be prevented by addition of alcohol(at low temperature).