It is a substance that, placed between two surfaces, is capable of establishing a connection, by following an adhering procedure. For that which a vast group of substances with adhesive properties may demonstrate good consolidating (and protective) properties, one tends to define adhesive action in relation to a surface, and consolidating action related to the complexity of the porosity of the surfaces. The numerous adhesives used in the course of the centuries in the pictorial technique and today in restoration may be classified in relation to the diverse chemical nature: one speaks of adhesives of animal origin (see group of entries for animal glue, casein glue and albumin glue), adhesives of vegetable origin (see group of entries for vegetable glue), including also natural and synthetic adhesives, elaborated natural adhesives, wax adhesives and mineral adhesives.