In my pre-plastic world there is a killer who removes unblemished skin from their victims and replaces it with a non-human 'skin', so that one can't tell the replaced skin from the remaining human skin.

What is the best candidate for the replacement material?

Ideally, it should fulfill the following conditions:

• Not made using crude-oil-based materials


• Available in most Caucasian skin tones


• Allows fine stitching to be applied


• May be fixed to or draped across a wire mesh


• Looks as similar to the remaining skin for at least 24 hours of partial exposure to the elements (moist European climate, dressed in appropriate clothing) as possible

EDIT: The skin is only to roughly mask the mutilation and serves no permanent function. The victim is dead and will not be moving in the foreseeable future.

Victims may have almost all skin replaced apart from the face and the palm-side of the hands. Younger victims tend to have more area replaced than elderly ones due to the growth of lesions and the likes.

I give you two choices depending on use:

Mortician's wax:

Mortician's wax is a cosmetic wax used by morticians to cover injuries
on dead bodies. The wax imitates the properties of human skin and is
used to fill in gouges, lacerations and areas where flesh is missing.
Makeup can then be applied to make the wax match the real skin.

Whilst the modern recipe contains petroleum based waxes, I see no reason why a beeswax - vegetable oil - animal fat and ground talc recipe couldn't be made that could be gently heated, applied with a palate knife, allowed to cool and then be stippled with a brush to simulate pores, then made-up to look like skin.

If it is required to be flexible and on a moveable subject the issue becomes more problematic.

Latex.

(Edit in response to comments: Obtainable from a number of plant sources worldwide: Dandelion, milkweed, rubber tree. - Found in 10% of all flowering plants.)

Latex can be applied, then made-up, but it tends to contract somewhat on drying if painted in thin layers and built up, it's more involved to give it a texture also: pre moulded latex can work very realistically.

Using a plaster of paris base, plasticene or fine clay if you prefer can be added and finely textired to make a "positive" (youtube video excellent tutorial - if you ignore the fact he's using a plastic base) This, when finished and dry would be coated with a very thin layer of beeswax or equivalent and then a fine spray of thin oil to act as a release agent.

A "negative" can then be made with alginate (seaweed extract - should have strips of fabric added to give it structural strength when it's peeled off) - when set, this is carefully peeled off and set to rest in a suitable pile of cloth downside-up, it can then be painted with latex. When dry this can be peeled apart, discarding the alginate, leaving perfectly textured skin-substitute ready for sticking on, and blending-in with make-up.

Pig skin

Pig skin is already used on humans as part of skin grafting.

It is generally only used as a temporary graft until a more permanent one is ready.

From a 1964 article, as the process was very new:

Pigskin dressings and immediate excision were both advocated for the
treatment of burns at the 49th Clinical Congress of the American
College of Surgeons in San Francisco in October.

Pigskin grafted onto patients with extensive third degree burns was
retained for more than two weeks...

In earlier experiments these investigators had used fresh
split-thickness pigskin on third degree burns in 200 mice. They found
that the grafts, irradiated or not, appeared soft and viable for
nearly three weeks. During the next two weeks the grafts slowly dried
and sloughed.

In 2012, Doctors graft pig's skin onto burned child. In this case it was done pretty quickly without a lot of processing. The skin was grafted and expected to take about 2 weeks to fall off as the child's own skin regenerated.

Here's a picture from a different article. It looks pretty decent (the last picture is a human leg with porcine skin). Maybe could use some makeup...

The closest you might be able to get would be prepared animal skin (cut, formed and dyed to the right specifications), but I sincerely doubt there is anything that will stand up to close inspection. If it only needs to fool from a distance, then that would probably suffice.

Gutta-percha was used in the times before plastics were invented, for pretty much similar purposes.

It became uncommon only after the mid-20th century, because plastics became easier to manufacture in larger quantities. Gutta-percha is made from the sap of trees, so it cannot compete in modern times with plastics due to economies of scale. However, it could theoretically be manufactured with medieval levels of technology.

Searching it among google images, there are many results in the colors you wanted.