The work ‘adobe’ comes directly from Spanish who in turn inherited it from the Arabic ‘al-tob’ (الطوب). The term simply means ‘brick’, having its origins in ancient Egypt as verified by surviving hieroglyphs.
Today ‘adobe’ has a more specific meaning of a sun dried brick formed from clay, sand, water, straw and sometimes having additives such as manure, soil, lime, etc. A similar mixture is used for mortar and as an earthen plaster to bond and protect the adobe bricks.
SCRATCH, BROWN, FINISH
This description of a 3 coat plaster system (usually over lath) is more common in the US and applies to all types of plasters: lime, gypsum, cement etc.
The ‘scratch’ coat is the base layer and as you might guess receives scratches to receive the next coat. It is important that the scratches run horizontal, essentially forming little shelves for the next coat to sit inside and lock into
The ‘brown’ coat is the middle coat. Traditionally either brown sand was used or some mineral tint was added so that the plasterer could easily gauge if he had sufficiently covered the scratch and to make sure he had good coverage when applying the finish.
The ‘finish’ is applied last. Unlike the first two coats where thickness was achieved and the geometry of the wall was established, the finish is typically a thin veneer to create a smooth surface.
|Image courtesy of Franco Saladino|
‘Marmorino’ describes an entire system of lime plastering inherited directly from the Romans as recorded by Vitruvius. It enjoyed a vibrant revival during the Renaissance, spreading from the Veneto region, where it had continued as a craft tradition, to the rest of Italy.
The word ‘marmorino’ is the diminutive form of the Italian ‘marmo’, meaning ‘marble’. So ‘marmorino’ has a direct translation something like ‘little marble’. Outside of Venice it has taken on the more specific meaning of the final coats which are rich in lime and taken up to a high polish
|Image courtesy of Simple Construct|
In discussing Adobe construction a few days ago we mentioned that the bricks traditionally would receive an earthen plaster. Earthen plasters are undoubtedly the oldest form of plastering because no cooking is required. Rather than having a chemical ‘set’ it simply dries out. They are still used in Adobe construction as well as over other natural building substrates such as Straw Bales and Rammed Earth.
The binding component of an earthen plaster is clay, meaning clay is the material that holds the other ingredients (silt, straw, sand etc.) together. A certain percentage of clay is required to make a suitable plaster. Too little and the plaster is weak and friable. Too much and the shrinkage of the clay will lead to cracking.
The image provides a good initial test to see if a site soil has a good percentage of clay or will need to be modified.
A common tool in the French plastering tradition. Wider versions are used to level wall surfaces whereas the narrow version is the tool of choice for creating faux masonry joints.
The narrow tool is called ‘Chemin de Ferre’ or ‘iron horse’. An appropriate metaphor as the plane has the shape of a train engine and runs along a straight edge or ‘track’.
|Image courtesy of Plâtres Vieujot|
Previously we discussed the running of plaster moulds on a bench to later be affixed. However, a more traditional method is to realize the work on site or ‘in situ’. This type of work is usually referred to as ‘run-in-place’ in the US.
The same mechanical process of running a profile along a track is used for run-in-situ as for bench running; however, the level of skill required is much higher. Instead of using Plaster of Paris which has a rapid set, a common mix is to use lime gauged with gypsum plaster and a small amount of retarder to provide more time to complete the moulding.
|Image courtesy of Palladio Mouldings|
Modeling describes the art of placing enrichments on a moulding in preparation to create a mould. Hence you’ll hear the expression ‘model and mould’ although they are two distinct actions not always performed by the same person.
A good modeler must understand layout well and is responsible for geometric enrichments. However, an experienced modeler will develop at least limited sculptural ability for repeated motifs such as egg-and-darts and acanthus leaves
|Image courtesy of Palladio Mouldings|
There is a distinction made between the responsibilities of the modeler and that of the architectural sculptor. Although still possessing a solid understanding of geometry and precedent in ornamentation, architectural sculpting goes beyond planting repeated motifs on a moulding to embrace free formed, often asymmetrical or unique designs such as cartouches, bas-relief or ornate column capitals such as the Corinthian capital being developed here.
Having been trained classically in figurative study and having worked in mediums as diverse as clay, plaster, wood, stone, the architectural sculptor often assumes the responsibility of art director working with his team of modelers in larger ateliers.
Contributed by Patrick Webb