Tanning development

As a developing agent, catechol tans the surface gelatin of the negative (see Tanning Developers, below). It is also easy to use catechol to compound formulas that create a desirable image stain density when low quantities of preservative (e.g., sodium sulfite) are used (see Image Stain, below). Both of these properties it shares in common with pyro. 

There are other developing agents, chief among them hydroquinone, that will produce a tanned image when used in a formula similar to D-175 (Formulas Extreme Compensating Developers D-175 Tanning Developer). Even so, pyro/cat are the two most commonly used for this purpose. 

It has been suggested that vegetable tanning developed from a desire to colour oil or alum-processed skins. Interestingly the earliest surviving recipes for the preparation of leather, dating from Babylonian times about 3000 years ago, show that a combination of these three processes were employed ... 

Erythrulose is a four-carbon keto sugar that reacts in exactly the same way as DHA. However, since they are different compounds, they do produce different melanoids with slightly different properties, including color. The tan produced by erythrulose is less reddish in tone than that produced by DHA. However, while a DHA tan develops in 2-6 hours, an erythrulose tan requires 2 days. Eor this reason, erythrulose is usually not used alone, but only in combination with DHA. 

---