Lime plaster

Lime plasters were common in the UK until the early 1950s. They can still be found in use today, although generally this use is confined to conservation work.

Lime is the binding agent in these plasters. It is created by processing limestone or chalk. Binders made from pure limestones and chalks, with a calcium carbonate content of 95 per cent or more, are known as air limes because they harden solely through absorbing atmospheric carbon dioxide – this process is known as carbonation. Air limes were commonly used for internal wet-applied plasters, where the necessary prolonged exposure to carbon dioxide could be controlled. Other limes were derived from limestones which have natural clay-based impurities. These produce a faster and stronger initial hardening. This occurs only once they are mixed with water; in fact they harden under water too. These ‘hydraulic’ limes were used as a binding agent in external and exposed situations where the slow, and somewhat delicate, carbonation process would have been compromised.

The creation of an air lime starts with the quarried and crushed limestone/chalk being heated to high temperatures in kilns; a process which drives off carbon dioxide. The material formed in this process is calcium oxide, known as quicklime. Quicklime is then slaked in water (a very vigorous and sometimes dangerous reaction generating very high temperatures) to form calcium hydroxide. The slaking action rapidly breaks up the quicklime into a fine, milky slurry. The excess water is then removed, leaving lime slurry with the consistency of thin cream. After a minimum of four weeks of storage the lime thickens up and is ready for use. It can, however, be stored indefinitely in the right conditions. Indeed, the longer the maturing period, the greater the stability of the binding agent. For air limes, the calcium hydroxide in this thickened and mature form is known as lime putty.

Hydrated lime (calcium hydroxide) is nowadays also bought as a dry powder in bags. This is an air lime in a dry form, produced when exactly the right amount of water is added to the quicklime, i.e. enough to convert the calcium oxide to calcium hydroxide, but not enough to leave a water residue, creating a lime putty. Hydrated lime is easier to obtain than lime putty and it can be more convenient. If used as a replacement for lime putty it is recommended that the dry powder is mixed with water to form a putty at least 12 hours before being mixed as a plaster.