Uses and misuses of alum in medieval and pre-modern artists’ recipe books

During medieval and pre-modern times, potash alum was used for a variety of purposes in artistic and craft contexts. Alum was one of the most important chemical compounds involved in the dyeing of textiles and the manufacture of organic lakes, used in painting and illuminating techniques. Knowledge on the various uses and functions of alum can be gathered by the studying of collections of recipes, of which hundreds of examples have survived from the Middle Ages and the pre-modern period. This study is based on a corpus of 450 recipes books mostly produced in Northern Europe between the 14th and the end of the 16th century.
Within these sources, alum is involved in a large field of technological procedures. It acts as a siccative agent in the preparation of oils and varnishes. It was employed in the making of glues, in the coloration and the treatment of various supports (bones, parchment, paper, wax, …), in the manufacture of pigments, such as verdigris, and the purification of ultramarine. This paper details the diversity of alum applications, as described within recipe books. It will notably focus on the varied roles played by alum in the preparation and conservation of a range of colorants which were typically preserved on small pieces of linen cloth and known under the appellation of ‘tüchlein’ colors.
In addition, through these collections of recipes we learn that alum not only served for artisanal purposes but it was also used in alchemical, medical, pharmaceutical and house-economical contexts. For example, alum is cited within alchemical instructions, as being involved in the producing of artificial vermilion or in the tinting and gilding of metals. It was also used for imitating gold and silver, and notably employed in the production of aurum musicum. In parallel to this initial information, recipe books also provide us with data regarding the physical qualities of alum, the diverse states in which it was supplied and possible substitutes. They also deliver some clues concerning its various geographical provenances.
Thus, these sources could serve to shed light on the multiple historical roles of alum but also help in accurately estimating its technical importance. Moreover, recipe books provide a source to investigate the multiplicity of alum appellations. Questions will be addressed such as: did these denominations reflect the plurality of uses of alum or were they related with the physical properties, quality grades or geographical origins of the alum product ? Finally this paper will try to establish the relevance of collections of recipes in reconstructing the history of alum. In so doing, it will consider how these sources can be related with other sort of historical sources and how they can illuminate each other.