Comparing vegetable tanning with conventional tanning with chromium, very often only the tanning process itself is compared.
However, this neglects the consideration of the tanning agent production, which surely has a significant impact on the ecological footprint of the leather too.
At the start here is some information about obtaining the chrome tanning agents and the chromium itself.
In the upper crust, the heavy metal chromium with a share of about 0.005% is one of the most common elements. Its pure form is rare; often it is found in complex combination with lead, iron and aluminum.
The most important mining areas with relevant resources include South Africa, India, Kazakhstan, Turkey, Iran, Russia, Brazil, Finland and a few other countries. Here is, due to socio-political circumstances often land expropriation, low-wage employment and overexploitation of nature on the agenda and the degradation causes permanent massive ecological problems.
The usually very poor indigenous population is hardly involved in the income and occupational safety or even social aspects find at best insufficient attention. In addition, the usually very costly renaturation concepts for the exploited areas are also missing, so that contaminated groundwater and surface waters harbor a considerable risk for the local population and the open-pit areas remain fallow for decades.
The extraction of chromium (estimated at 31 million tonnes in 2017), like other heavy metals and rare earths, also generates great quantities of useless waste, so that a complex and resource-sapping treatment technological process for the production of chromium is necessary.
In this case, the soluble chromium compounds are separated from the iron at high temperatures of about 1,000 ° C within an oxidizing reaction.
The resulting chromate is transferred with sulfuric acid into the dichromate and then reacted in a further process to chromium (III) sulfate. The latter compound is then used in tanneries for chrome tanning.
Compared with the regrowth of the rhubarb root, the resources required to produce the approximately 620,000 tonnes of chromium tanning material required annually (about 2% of the total) are many times higher and the environmental / social consequences in the mining areas are often tragic.
Rhubarb is a renewable resource that grows relatively undemanding in almost all regions of the world and produces a lot of biomass; depending on the planting density between 40 and 80 tons per hectare. As a result, even with large quantities of leather, no agricultural land will be lost to food production.
The extraction of the vegetable tannins is technically very easy to implement and the required extractants are considerably less polluting than the chemicals used in the production of chrome tanning agents.