Composition and Technology of the 17th Century Stucco Decorations at Červená Lhota Castle in the Southern Bohemia

The stucco decoration from the Great Chamber of the castle Červená Lhota in the Southern Bohemia was surveyed and characterised as part of its planned conservation. The repair works that are still to be specified in detail should be based on a good knowledge of the original material and technique. The main focus was on the fruit pieces decorating the ribs of vaults. Altogether, over twenty mortar samples were collected from the vault bedding mortar, plaster, drawn elements and stuccos. Additionally, some pieces of fruits, that were loose and had to be temporarily removed, were studied macroscopically and by X-ray CT. Characterisation of binder, aggregate and additives was carried out using TA, acid attack, sieve analysis of unsolvable residue, XRD, FTIR and nL-MS. The characterisation led to understanding of the uses of various mortar mixes and determination of their recipes. A typical fruit piece was composed of an inner modelling core mortar made of lime-gypsum binder and a coarser aggregate covered with a 5–20 mm think lime rich stucco layer containing calcitic air lime and fine siliceous sand. This stucco mortar was also used for modelling various small decorations - fine and thin shapes like leaves, cherries and grapes. These were attached to the main piece by a fine adhesive mortar made of lime and gypsum in various proportions. The use of gypsum and the differences in lime to gypsum ratio corresponded to a certain functional requirements (performances). This finding was a key parameter that contributed to the reconstruction of the original application technique and procedure. Qualitative analysis of animal proteins determined presence of collagen – an animal glue that was used to modify the setting of gypsum and which was also used in the final finish. The results are discussed as an example of a procedure that aims at a detailed replication of the original technique.