Did the Indus Valley Civilization use cotton as a writing material?

Question

Cross-posted from Skeptics SE

In his book, Science and Secrets of Early Medicine, Jürgen Thorwald writes about the script of the Mohenjo-Daro and Harappa civilizations: (in rough translation)

Seals, bronze plaques and clay shells show that the inhabitants of the Indus state had their own script, which was a pictorial script, just the earliest scripts at the Nile and in Mesopotamia. (...) Undoubtedly, there used to exist a lot of writings created using this script. However, while Egyptians used papirus and the Mesopotamians used clay tablets for writing, the ancient Indians used cotton and perhaps also tree bark. No one knows it for sure, because the tropical climate probably destroyed everything that the former libraries and archives contained. Since all written documents were destroyed, our knowledge of the culture of the Indus valley civilization relies on archeological findings.

(empahsis added)

I was able to confirm that the Indus Valley Civilization had its own writing system and that it was used on seals and pottery. However, none of the resources I found said anything about using cotton or bark as the writing material, and this History.SE post actually suggests the contrary.

Do we have any reasons to believe that the Indus Valley Civilization had extensive written documents and that they were lost due to the climate and the material used, as the author suggests?

Answer 1

It looks highly unlikely. All we can say for sure is that there doesn't appear to be any evidence that such a thing ever happened during that era.

The vast majority of our exemplars of Indus Valley Script were made on pottery or engraved into stone, particularly stamp seals. Of those, the majority appears to have been on pottery. However, I've also been able to find references to the script being found on tools, tablets and ornaments. Host materials reported to have been used are terra-cotta, ceramic, stoneware, glazed faience, shell, bone, ivory, sandstone, steatite, gypsum, copper alloys, silver, and gold. Intriguingly, some of the exemplars on pottery appears to be post-firing graffiti^*.

Of course Thornwald is right to point out that this was so long ago that any biological host material would almost certainly have not survived. However, he should have stopped there. Absence of evidence may not be evidence of absence, but it is certainly not evidence of presence!

The next question might be, did they have access to cotton? Well, yes they did. Cotton cultivation in the Indus valley system actually pre-dates the Indus Valley Civilization and its script.

Where the problem comes in is that our first actual evidence of writing on an organic material in that part of the world doesn't come for more than a thousand more years (500BC). When it comes, its on palm leaves, not paper or (cotton) rag-paper. Paper wasn't widely used on the subcontinent as a host medium for writing for more than another millennium after that (sometime between 700AD and 1000 AD).

So there doesn't appear to be any good evidence backing up that claim of cotton-paper writing in the Indus Valley script.

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^{* - Its tempting to speculate that this unusual use of pottery indicates that it was the cheapest writing material available in that area at the time, which would be evidence against any kind of writing paper existing. That's just me spitballing though, and there are probably other possible explanations for that quirk I haven't thought of.}