A very popular sight in Delhi monuments especially of the Lodi Era is beautiful incised plaster work on niches and other surface decorations.
The most spectacular of these is of course the colored ceiling of the tomb of the 16th century Sufi saint Jamal.
Visual Research on 5 Delhi monuments by Divya Rani and Dr. Santhanam of Indian Institute of Technology (IIT) Madras, which hosts the National Centre for Safety of Heritage Structures has found that this historic plasterwork depicts that these are made of two layers; a base coat which is coarser in appearance and a finer finishing coat, which is the incised plaster layer. For making the intricate patterns in the incised layer, a thick coat of plaster is applied first and patterns are carved out before the initial set of the mortar, which later hardens to form a very dense microstructure.
Their test results indicate that the incised plaster is made of air lime binder and siliceous aggregates. The mortar is characterised by a very narrow range of pore sizes of about 600 nm. The use of organic fibres and inclusions from an older mortar are also evident in the analysis.
The results showed that the plasterwork was mostly made up of lime binder and silica sand. Interestingly, organic fibres were also found in the plaster. Divya Rani, a Ph.D. scholar at the institute and first author of the study published in Construction and Building Materials, explains that the organic fibres could be anything from jute or straw to animal hair.
Plaster is prone to shrinking and fibres prevent the formation of cracks. Fibres are also used in modern construction to prevent cracks in concrete.
From an abstract of their article and interviews in The Hindu