Posted: 05 January 2010 at 12:51pm | IP Logged

The use of most leather chemicals in the tannery has an inherent contradiction that can be avoided by controlling astringency between the added chemical and the relevant reactive group on collagen.  Fortunatelly, at least the initial chemical reactions leading to much more complicated processes, can be broken down into manageable conceptual situations.

The factors that encourage good fixation of a particular chemical offered to Collagen/leather usually imply that under those conditions very poor penetration occurs.  Thus a tanner must apply initially low astringency (chemical reactivity) conditions untill the desired penetration is achieved and then switch the medium conditions to high astringency situations.  In general the concentration of a chemical causes better penetration through diffusion, if astringency is low, and thus low floats will in general help penetration.  If penetration is not desired such as when top-dyeing, the dyestuff can be added at a pH and temperature causing higher astringency and in a large bath such as to hinder penetration.  In the case of vegetable extract tanning in a pit process where astringency is not controlled very well, only the offering of very dilute initial concentrations of the extract during a very long time, achieves sufficient penetration.  Unfortunatelly this fossil concept is still being carried through and applied to drum processes where astringency can, and needs to be controlled, if the basic chemistry is understood!

Many processes such as dyeing are initially affected by longer range electrostatic inverse square forces, but the really effective fixation is ultimatelly defined by much shorter ranged, molecular chemistry distances  linkages such as coordination, H-bonding, van der Walls, hydrophobic, Dipole-dipole, etc. shorter range forces that only take place after the initiation of the drying when dye-collagen internal structural distances are shortened to the point that the closer ranged molecular chemical linkages beguin to occur over and above the initial electrostatic attractions.  Thus, in general, dyestuffs only effectivelly really fix during drying of the leather.  Thus the chraftsman's folklore general rules of thumb such as adding an equal amount of fixing formic acid as the weight of the dye employed, are only based on faulty understanding of the chemical situation!  Enough formic acid must be added to create sufficient number of charged aminos on collagen for the initial attraction, thus the internal pH of the leather at dyeing leading to charged aminos forming is the important factor, even if difficult to estimate with pH indicators because of the colored dyestuff!

Posted: 07 January 2010 at 7:30am | IP Logged

Interesting that you would use dye as your example.  Personally I think the picture is clearer using the chrome example as there is clear separate stages of covering, exposing and fixing the chrome to the protein through the pickle, tan and neutralization.  I often emphasize that the neutralization is the true tanning stage, not the "tan" or simple addition of the chrome liquor. 

However, the point is well taken if somewhat muted in presentation.

Posted: 07 January 2010 at 9:53am | IP Logged

I do not want to get into the theory of tanning, but the inorganic chrome hydroxide extended polymers only form after increasing the pH to nearly precipitating Cr(OH)3-they are the ones that are involved in tannage!

Dyes interact with chrome tanned leather initially by the long range electrostatic attraction of their negative sulphonic (solubilizing!) groups and the cationic aminos present on collagen whose availability is not only pH-controlled but also from the decoupling the "other-end" of a saline link, that is then set free by metal tannage, causing their decoupling.  Thus metal tannage makes collagen/leather more reactive to anionic chemicals, that is "more astringent".  That is why I mentioned dyes in the posting.

Posted: 25 January 2010 at 3:33am | IP Logged

Solubility of Ca and Mg in plain water decrease when the temperature increases. But in the tanning MgO is added in acidic conditions and it is more soluble there than in plain water. If the MgO is not well prepared and its reactivity and solubility is not reduced by heating and its particle size is too small, then it is readily soluble and when added it gives a jump in the pH of the tanning bath increasing instantly its value for more than one unit and this precipitates the chromium giving final leather with a green color on the surface, not the usual bluish shade. The MgO specially made for basifying is not readily soluble by heat treatment and controlled particle size, but it needs a final temperature around 40ºC or higher in order to be completely dissolved. This special MgO increases the pH in a very smooth curve as the bath temperature is also increasing.