I've always been fascinated by the idea that 'everything has a use' the innate inventiveness of humans allowed our ancestors to turn even the most unlikely ( and unappealing) parts of plants and animals into useful and often beautiful items. I frequently pick up Roadkill animals so that I can make use of the skins, bones and other parts, which gives me access to materials such as stomachs, intestines, bladders which usually get left at the kill site by hunters- it makes the carcass lighter, and saves disposing of them.
Intestines, stomachs and bladders have an interesting structure- the inner layer ( the mucosa ) has to be removed, along with traces of chewed food. the remaining layers are thin, but fairly strong, thanks to perpendicular layers of collagen fibres and muscle. Picture shows the inner mucosal layer being removed from the 'honeycomb tripe' section of a deers stomach. ( Deer are ruminants and have 4 chambers; The rumen ( the biggest- good for waterbags, even for cooking bags) the reticulum ( shown in the photo, known as honeycomb tripe) is beautiful and can be used decoratively. The other two ( Omassum and abomassum are of limited use but have interesting textures of their own- more experiments needed!)
The resulting material is then usually inflated ( or pinned out) so that it can dry without shrinking. In this state they can be used as fishing net floats, along with fish swim bladders. Innards are naturally waterproof- which makes sense when you think about their job in a living animal. Though they will seep gradually and have been used as water bags for millennia.
Smaller organs such as the bladder ( and even the pericardium- heart sack) are smaller and make good breathable but waterproof containers for tinder and greasy concoctions like lamp oil, or medicinal salves. We have no evidence for such usage in Paleolithic Europe, but Inuit peoples have been using intestine from large sea mammals to make waterproof parkas for centuries. They are incredible works of art and vital to a people whose lives revolved around the Sea.
Dried, the organs can be used as they are, but tanning in natural bark tannins makes them stronger. Tannins can be extracted from a huge variety of tree barks and leaves ( even fruits such as acorns) Oak, oak galls, walnut, alder, sweet chestnut, spruce, cherry, willow, cheap nasty builders tea.... It also helps to get rid of any residual smells and usually imparts a lovely colour ( brown, to orange to black)
Sewing such a delicate material takes time and patience and a few ingenious techniques. For making a waterproof parka, the stitches need to be almost impossibly small.- One of the experiments I want to do is to try sewing tanned cow intestine- (the closest I can get to whale!) with bone needles and sinew, and try other methods that might have been available to our paleolithic ancestors- such as awling holes and poking sinew through, or using a boar bristle as a needle. If you simply make tiny stitches, they will pull through or tear the material without much provocation. So the techniques used have to spread the load of the stitching- using folded over edges, welts, reinforcing strips and by sewing over a cord or strip of grass sinew/ cordage. I will look at these in more detail in another post. I wrote a more detailed article for the Buchcraft Journal magazine on tanning tripe; https://www.thebushcraftjournal.com/product/the-bushcraft-journal-magazine-issue-30/