The salt concentration is usually critical in preventing acid swelling during the pickle.TFL has recommendations for the precise control of the key process.
To avoid acid swelling the first action of pickling is to adjust the float, normally 50 - 80% depending on type of skin or hide, and add salt. While lower salt concentrations may control acid swelling, in practice to avoid problems, a minimum Baumé of 5° - 8° must be aimed for.
This salt solution should be given time to mix and to reach equilibrium in the pelt and then bechecked again for Baumé level before adding any water and allowed to cool before adding to the vessel. Do not forget to allow for the water used for acid dilution affecting the final Bé. To prepare the skin for chrome tanning the pH in pickle is usually adjusted to ca. 2.5 - 3.0 depending on the tannage and the type of raw material.
When formic acid or other products are used to help pickle penetration and as masking agents they should be added before the sulphuric acid and run for at least 15 - 20 minutes to distribute.