In answer to Paul's request about 'touch pieces':
Really very straightforward; it's buying, and holding onto, a known piece from a given company or maker to use as a guide/comparison to others that you might later buy. If all the characteristics match then it it is reasonable to suggest that the new piece is also from the same company/maker. Although, you have to be very strict with yourself to not persuade yourself something is what in fact it ain't.
In effect pretty well all collectors are doing it all the time without necessarily realising.
I guess I do it to learn, particularly about more obscure companies. For instance I bought a wonderful piece of Arculus recently. It is known through one of the photographs in Charles Hajdamach's
20th Century British Glass. Not only will it help me determine the characteristics of that company, but it may help with comparing to Walsh Walsh 'Pompeian' pieces to discern any differences.
I've done this for many years, and found it particularly useful to understand early Powell Arts & Crafts glass before both the books and exhibitions of the mid/late 1990's
If you're lucky the 'touch piece' is cheap, but in order to learn it is always worth pushing the boat out IMHO

PS. The Arculus piece is a different colour to those shown in Charles's book, which I guess helped ........ who
really wants brown?? I'm hoping it will help with some unknowns on that big 'shelf' that we all seem to have!!
Hope that answers the question, Cheers, Nigel