Palladio used iron gall ink, which was also popular with artists in the Renaissance period. Iron gall was popular as it is hard to remove from the paper's surface once drawn with.
Palladio used iron gall ink for sketching free hand or to go over initial faint chalk, graphite, or a lead underdrawing. This type of ink has been used in all of the drawings by Palladio in the RIBA collections.
Iron gall ink was made from iron, tannic and gallic acids and the quality varied enormously. A typical recipe might be: soak oak apple galls, or other type of galls, in water or wine for up to 6-8 days, strain the extract and then add ferrous or copper sulphate. The best were Aleppo gall-nuts from around the Mediterranean.
The ink would be exposed to the air for several days prior to use to make it turn black, as initially the colour would have had a grey to violet tone. Once applied to paper, the colour would change from black to brown over time. Bistre and sepia inks are always brown.
Palladio used both diluted and undiluted washes of the iron gall ink to create depth. A solution of gum arabic was often added to limit both the penetration into the paper and feathering of lines, problems that were often associated with a different type of ink known as bistre.
Iron gall ink did not produce as black a tone as the concentrated carbon inks of today and, unlike other inks, the tonal values were produced by a chemical reaction.
Sheet XI/15 clearly shows the range of tone iron gall ink could create, from very light brown washes to the much darker, richer brown ink spill and inscriptions. The use of iron gall ink declined in the 20th century due to the introduction of synthetic dyes.