Thilo has been studying archaeological materials using methods developed for mineralogy since the late 1980s, first concentrating on technical ceramics and their relationship to the increasing complexity of metallurgy from the Early Bronze Age to the early modern period. This led to fundamental work on cupellation, that is the separation of silver and gold from the base metals copper and lead, in large-scale production and in the small-scale workshop. From the latter, work on early chemistry developed, which has been taken up and further developed by his former student Dr Marcos Martinon-Torres. Related to this is also a series of studies on medieval and early modern metal smelting, from lead and silver to brass and gold, in the Old and New World.

During the investigation of a set of bronze-casting crucibles from the Late Bronze Age capital of Ramses the Great in Egypt Thilo discovered the first hard evidence for glass-making in that period, jointly with the excavator, Dr Edgar Pusch from the Pelizaeus-Museum in Hildesheim. This led to a paper in Science in 2005, and a host of other peer-reviewed publications. Based on the experimental and analytical work of several of his PhD students, including Dr Satoko Tanimoto and Melina Smirniou, we now understand LBA glass-making in much more detail. Subsequently, Thilo has also made significant contributions to the study of Roman glass, in collaboration with colleagues and PhD students from Bulgaria and Jordan, such as Dr Fatma Marii and Anastasia Cholakova, and the investigation of the formation of the earliest Chinese glazes, jointly with his PhD student Min Yin.

At the same time, Thilo has returned to an earlier interest in arsenical copper and its production and use during the Early Bronze Age; here, he has recently published work on the smelting of speiss, an iron-arsenic alloy, based on work of Chris Thornton and Vince Pigott in EBA Iran. Related to this interest in early metallurgy is also his work with Miljana Radivojevic, a current PhD student, on the earliest evidence for copper smelting, world-wide, around 5,000 BC in Serbia.

Other research includes work on the production of crucible steel in tenth century AD Central Asia, the source of the feared and admired Damascene steel that played a major role during the crusades, and stimulated much basic research into the iron-carbon system in the early stages of metallurgy as a material science.