In Covington’s (2011) opinion, aluminum, titanium and zirconium cannot match the tanning power of chromium.Copper(II) has a complex chemistry with oxy ligands, but with a tendency to form complex anions, which are not useful for tanning. And all metal salts can be used together in all proportions. The benefits of introducing another metal salt can be summarized as follows: (a) the efficiency of chrome uptake is improved by reducing the offer (FutureTanning Chemistries, 2000); (b) the rate of chrome uptake can be improved by the presence of another weaker mineral tanning agent, applied as a pretreatment (Simoncini et al.,1978).