Pewter-work, in the form of goblets, clocks, tankards, wine carafes, trays, and salt- and pepper-shakers, is a luxury side-product of the tin industry.
Pewter-work are the cheapest items produced by the tin industry. 
mnli hypothesis: Pewter-work are the cheapest items produced by the tin industry.  premise: Pewter-work, in the form of goblets, clocks, tankards, wine carafes, trays, and salt- and pepper-shakers, is a luxury side-product of the tin industry.