A book link (French language) about the work of Diop. So I can find it again.
A bit unkind about Diop’s ’melanin drop test’, observing that they can’t find any evidence that the tests were carried out in a scientific manner that would make them accurate, and that the surface skin of the mummies is a poor guide to the in-life colour of the skin, as they are turned brown by the natron bath and then often coated with bitumen (which is black). It also observes that the reason Ramses skin tone turned yellow was not the irradiation, but because they had to give him a good clean to remove the mould that had invaded his skin, and this removed some of the dark coloured preservatives that had made Diop belive he was black.
A short translated piece…
The claim that egyptologists made the specimens with the most Negroid traits disappear is not only insulting to archaeologists but also put forth without any evidence and little practical credibility because all materials exhumed in an excavation are systematically recorded before numerous witnesses. It is equally fanciful to claim that the mummy of Ramses II, which was the subject of thorough studies in Paris, became yellow after being irradiated. Tests had been done previously on diverse mummified tissues to make sure that irradiation, the only way to sterilize the colonies of mold that were invading the skin, would do no damage. If the skin tone changed it was solely due to a cleansing of the surface. In fact, most of the mummies are coated with a tar (bitumen from Mesopotamia), which seriously constrains their examination, by the naked eye.
This is why a histological examination of skin to measure the amount of melanin is necessary. Unfortunately Diop’s (1973) article sheds no light on this. In fact in twelve pages of text and sixty references only three lines deal with the results: “We can affirm that such an examination reveals, with no doubt possible, an amount of melanin which is unknown among the “leucoderm” races and which indubitably places the ancient Egyptians among the Africans of Black Africa.” This is followed by long digressions on prehistory as well as biochemistry in a scientific jargon, which is highly documented but fails to mask the absence of results. No histological illustrations, a sketch of quantification, comparative examinations of skin with different stages of tanning, or reference to other studies of mummies (European, Peruvian, etc.) are provided. Cheikh Anta had photographs about his work circulated at the Cairo Colloquium (UNESCO 1980, p. 799); unfortunately no trace of these remains in any publication and it should all be repeated more rigorously. The Rabino-Massa team (1972, 1981) went much further, but skin of the mummies, unlike the internal tissues, is often altered by baths of the preserving liquids. Moreover, as Szabo (1975) points out, “light microscopic sections from a dark Mediterranean skin can be very similar to those from a Negroid skin,” thus more refined techniques such as electronic microscopy should be employed”
It also mentions that Diop’s work was published before the modern science of DNA analysis proved it was biologically impossible for modern Egyptians to be historic immigrants to the Nile valley (based on Y chromosome evidence) and points out the amusing mistranslation of the Egyptians being ‘black’, from Herodotus. The Colchians, who Herodotus described as being like the Egyptians, were a population up by the south of the Black Sea, and had zero likelihood of being literally black skinned. A booklink for a more in depth explanation, here.