Technical Advice TAN 1
Lime Putty used for the production of lime mortars, plasters, and renders must be of the best quality and the ageing or ‘maturing’ of non-hydraulic lime putty is an essential part of the production of best quality material. Lime putty should be properly matured for a minimum of 6 months, and longer maturation periods are necessary for many conservation applications.
Three processes occur during maturation:
- Any ‘slow to slake’ particles of over-burnt or partially converted quicklime remaining after the slaking process can continue to slake during maturation in the pit or porous container.
- Water is lost by drainage and evaporation and the density of the putty increases.
- Lime crystals (portlandite) undergo important morphological and dimensional changes during ageing which results in improved plasticity, workability and water retention. The crystals reduce in size but also change shape becoming ‘platelike’ or flat. (Getty Conservation Institute 1998)
The maturation of non-hydraulic lime putty is not the only factor in the production of best quality material, and maturation alone does not guarantee best quality.
The following should therefore be noted:
- The proportions of quicklime and water during the slaking process must be carefully controlled. A temperature of between 95°C and 100°C should be maintained in the material itself during slaking. The lime will not achieve this temperature if too much water is used, and ‘drowned’ putty is an inferior material. There is a danger also of “burning” the lime during slaking if insufficient water is used, and too high a temperature has a tendency to coagulate the minute colloidal particles and diminish plasticity.
- The putty must be matured in a porous pit or vessel (not a plastic tub) where excess water is free to drain and/or evaporate. It must also be protected from frost during this process.
- The density of lime putty should always be checked as this gives a good indication of consistency and quality. Lime putty should have a density of at least 1.35g/ml and 1.40g/ml is indicative of good quality material.