Tin cans as portable antiquities

I came across an archaeological text from Indiana USA called "HOW OLD IS "OLD"? Recognizing Historical Sites and Artifacts" (Sharon A. Waechter et al.) which made weird reading for a British archaeologist. I once dug up a biscuit tin in the fill of an old excavation trench at Wroxeter which was identified and dated by the manufacturers and which made it clear that the trench was probably one dug by Kathleen Kenyon, who therefore had gone deeper than she admitted in her report, but in general I'd never given rusty old tin cans much thought before. Whether a site was pre-1960 mor post 1960 had never really meant much to my own work. I was surprised to read that the authors consider "Ceramics in general are less time-sensitive than cans and bottles".