Not even the smallest admixture of alloy was permitted in the gold coinages of the Middle Ages. Dante (Inf. XXX. 73) mentions the coiner Maestro Adamo who had been burnt at Romena in 1280 for issuing florins which had scarcely more alloy than a modern sovereign.
The gold penny of Henry III. and the florin and its parts of Edward III. were only struck as patterns. The gold noble was first issued in 1341; its value was 6s. 8d. or half a mark. So many nobles were destroyed to make gold leaf for illuminating, and for other purposes, that an Act was passed prohibiting, under severe penalties, the use of the gold coinage for any except monetary purposes.
In the same way the gold leaf used by the Greeks was comparatively thick. The famous Erechtheum inscription of 404 B.C. gives one drachma as the cost of each leaf (πέταλον) used for gilding the marble enrichments; see Cor. Ins. Att. I. 324, fragment C, col. ii. lines 35 and 42. Eighteen-pence will now buy 100 leaves of gold.
The best account of the way to make the mordant was given about 1398 by a Lombard illuminator called Johannes Archerius; see Mrs Merrifield's interesting collection of Treatises on Painting, Vol. I. page 259 seq.
See Theophilus, I. 25.