Skuse testified as to his conducting Griess tests upon the hands of the six men following their arrest, stating as to his being 99% certain that both Hill and Power had handled explosive materials, and to a possibility Walker may also have done so. Skuse conceded that he could not rule out the possibility that Walker's right hand could have been contaminated from his (Skuse's) own hands, as Walker was the last of the five men to be swabbed at Morecambe police station, and had at first tested negative to the Griess test, before a second swab had revealed faint, positive traces of ammonium and nitrates. This testimony was refuted by Dr. Hugh Kenneth Black, a former Chief Inspector of Explosives for the Home Office, who testified that a range of innocuous substances and objects one could handle on a daily basis containing nitrocellulose (such as varnishes and paints) would produce a positive result to a Griess test. Moreover, the tests conducted by Skuse had not succeeded in identifying nitroglycerine as the source of the positive results produced by the Griess tests, and the Crown had earlier conceded that an exhaustive search of the six men's homes had revealed no traces of nitroglycerine.