I have now taken 115 samples of black goo and yellow varnish from 17 coffins, mummies, and mummy cases (cartonnage). The ingredients of the black goo are very similar to mummification materials, mainly comprising of plant oil, conifer resin, beeswax, and bitumen, although the exact recipes vary. Above is the wrapped mummy of Denytenamun (EA6660), which was covered in thick black goo.
Cartonnage containing the mummy of a girl (approx. 7 years), named Tjayasetimu. EA20744 (acquisition 1888).
Curators in the Ancient Egypt and Sudan department at the British Museum are currently putting together a new, updated, catalogue of the mummies, coffins and cartonnage from the 22nd Dynasty (c. 900 - 700 BC) that are in the collections of the museum. Cartonnage is made from linen and plaster layers, like plaster of Paris, and usually formed in an anthropomorphic shape, as in the photo above; the mummy is inserted into the case and it is laced up at the back.
Alongside this there is a project in the Scientific Research dept to analyse the organic deposits / coatings that can be found on many of the coffins and cartonnage. These fall into two main categories: yellow varnish, and what I'm calling "black goo". The latter was seemingly poured over the cartonnage after it had been placed in the coffin, and some examples are cemented into the coffin with this substance, which dries to a hard, friable finish (below). There are also black paints used on the outside and inside of coffins, both as part of the decorative scheme and as a more total black coating, these may be different materials again.
Polychrome and gilded cartonnage containing the mummy of Djedkhonsiufankh. EA6662 (acquisition 1834).