The historical manufacture of gold leaf, known as goldbeating, has not changed much since ancient times . By hammering a lump of gold, a thin gold plate or ribbon is formed; the gold plate/ribbon is then cut into squares and placed within a stack of paper or parchment, and repeatedly beaten until it reaches the desired thickness . In the medieval period, coins served as the most common material source for gold leaf [7, 8, 9]; especially from the mid 13th century, gold coins with stable, high purity such as florins (23.75–24 carat recorded in 1252) and ducats (23.75 carat recorded in 1284) became a reliable source for gold leaf . Evidence of the use of such high-quality gold coins is traceable in some work contracts and guild regulations. For example, in the statutes of the goldsmiths’ guild, the Arte di Por Santa Maria, of 1335, the gold used for artworks is required to be sourced exclusively from florins .
The leaf thickness is of great significance in gilding history and denotes the level of metallurgic and goldbeating technologies of the epochs. Nevertheless, the selection of gold leaf sometimes has to face a dilemma: on one hand using thin leaf is critical for artisans to keep a lower cost during the creation process of artworks; while on the other hand a moderate thickness provides mechanical stability, since very thin gold leaf can be easily torn during the burnishing process or worn out. Therefore, some medieval guild regulations and artists’ treaties particularly defined a required leaf thickness by specifying a required number of gold leaves produced from each gold coin. For example, a Florentine guild statute fixes a rate of one florin to one hundred gold leaves ; Cennini also suggests 100–145 sheets of gold leaf from a single ducat . In the latter case, combining the weight of ducat (ca. 3.5 g), its gold purity (23.75 carat) and the common sizes of gold leaf in the 14th century’s Florence (6.8 × 6.8 cm and 8.2 × 8.2 cm ), the leaf thickness is calculated to be in the range of ca. 190–400 nm. Kubersky gives an even thinner and more specific number of 154 nm for the gold leaf of the 16th century based on Vasari’s treaties . The National Art Gallery of London  provides an average thickness of medieval gold leaf of around 2.69 µm based on the calculation with the given weight of the florin and the number of leaves per coin, which seems significantly thicker than the historical records, indicating the presence of diverse specifications for medieval gold leaf. Indeed, scientific analysis of artefacts has shown that the thickness of medieval gold leaf varies from 100 nm to a few microns [6, 12, 13]. It is interesting to note that while each of these estimates describes the produced gold leaf as having a very regular square or rectangular shape (in agreement with observations), there is no mention of the offcuts necessarily produced from trimming the leaf edges and so the thickness estimates provided above should be considered as an upper bound.
A special variant of medieval gold leaf, called “or fin renforcie” (fine reinforced gold leaf), “d’or fin doubles” (double weight fine gold leaf) and “or doubles renforciez” (gold leaf, double reinforced), which was mentioned in records of the market price of artists’ supplies in Dijon in the turn of 1400 , deserves some attention. The two former types were double, and the latter was 1.5 times the price of a normal gold leaf, suggesting a respective leaf thickness of 2 and 1.5 times thicker than the standard. Indeed, the use of gold leaf with different thicknesses is recorded in medieval treatises, for example, Cennini suggests using thicker leaf (e.g. 100 or less sheets produced from one ducat) for flat backgrounds, as it is more suitable for burnishing, and thinner leaf (possibly 145 sheets per ducat) for mordant gilding and frame moulding . Other medieval treatises such as De diversis artibus by Theophilus (1115–1134) and the Montpellier Treatise (14th – 15th century) also mention that 2 or 3 layers of gold leaf can be applied through glair if desired .
In addition to the leaf thickness, the lateral size of the gold leaf is also an important technical specification of historical gold leaf. It is also a critical factor for the traditional method of estimating or calculating the thickness of historical gold leaf, as discussed above with examples of Florentine gold leaf. However, literature records regarding the leaf size and format of medieval gold leaf present a great deal of variety. For example, gold leaf in northern European countries from 1390 till the Reformation is reported to be in either square or rectangular formats with one side of 80–93 mm , while other researchers state that gold leaf in northern Germany and Sweden has the sizes of 9.6, 7.2 and 6 cm square . In southern European cities such as Florence, 6.8 × 6.8 and 8.2 × 8.2 cm are recorded for gold leaf in the 14th – 16th century , while in the painters’ statutes of the Florentine Guild of 1403, the size of gold leaf was fixed as one-ninth to one-eighth of a braccio fiorentino per side, corresponding to ca. 6.5–7.3 cm . Therefore, the calculation or estimation of gold leaf thickness can be only performed based on reliable literature sources from specific regions and epochs.