6) Sheets of paper could be thick or thin with different grains. Many forgers have used old sheets to produce works so as to make believe they had been made during previous centuries.
7) The signature of a painter is not a guarantee that the work being examined was produced by him. Many forged signatures have been added on oil works, drawings or watercolours. The same objection applies for prints. For example, the printed signature of Salvador Dali was often added on white sheets of paper even before they went to the lithographer's studio or worse, such signature or even a forged one, was added to lithographs. There were also some so-called posthumous signatures added on certain prints (Magritte for example, by his wife) by the heirs of many artists.
8) Techniques : There have been different working techniques : oil, tempera (a medium containing egg), acrylic mixtures, watercolours, gouache, crayon, black or red chalk, plumbago, monotypes and prints of all kinds used by artists.
9) Reproductions or prints usually carry little value though some genuine engravings produced by famous artists, such as Dürer,Cranach, Altdorfer, Rembrandt, Goya, Piranese, Daumier, Degas, Géricault, Toulouse-lautrec, Nolde, Munch, Picasso, Matisse, Renoir, Bonnard or Morandi could reach tremendous prices.
Regarding old prints it is however important to have the opinion of an expert as works were often produced step by step with trial copies, first state, second state, third state and so on, notwithstanding the fact that original plates were often re-used well after the death of an artist and sometimes for many centuries as in the case of Dürer, Altdorfer or Rembrandt. One important factor is the paper and often its watermark as many people wrongly think that being in a possession of a rare print would enable them to strike gold.