We recently looked at the various accelerators and retarders available for plasterers working with gypsum, cement and silicone based products, but what about lime based renders and mortars? Well, as these are the predecessors of modern plastering products there are a multitude of things that can be added to lime plasters and mortars to change their properties and drying times. The Romans were influential in developing different mixes for different purposes through trial and error, using blood, milk and fats in their lime plasters to achieve certain effects that were needed in different applications. For example, the mortars and renders used to build aqueducts needed to be as water resistant as possible. These structures needed to last for many years and had constant exposure to water.
As late as 1805, blood was still being used as a plasticiser and water repelling additive; Thomas Telford used it in the lime mortar used to construct the Poncysyllte Aqueduct, to ensure it would weather well and not degrade from the exposure to water. Times have changed and blood is no longer being used as an additive, but there are still some more savoury options available to those working with this traditional building material.
Sodium sulphonate, usually in the form of coconut oil soap, is used to plasticise and add water repelling properties to a lime plaster mix. The addition of the soap traps air bubbles in the lime, which remain in the dried final finish, helping it to repel water and stop the ingress of water into the wall behind it. The lime plaster is also made more flexible, aiding slightly with application, but ensuring that the product does not shrink and crack as it dries, leading to further problems with water getting in.
Fats contain substances called stearates (which are also found in shampoos, soaps and other personal hygiene products) that add a water repellent property to lime mortars. Acting like the surface coating of waterproof wax on an outdoor jacket, the stearates reduce the surface tension of the lime mix. This causes water to bead up and run off the surface of the wall (this is called the lotus effect) rather than sit on the lime and potentially be absorbed. Although nowadays we might use a commercially available product rather than animal fats, olive oil and similar types of fat have the same properties and would have been used by the Romans to create water repellent mortars and renders.
Accelerators are not typically used with hydraulic lime renders, as the product needs at least 72 hours to dry; any shorter and the hydraulic set is not achieved and the job must be done again. Retarders are useful therefore, to ensure that the surface stays wet for long enough to create the hydraulic set properly. Methyl cellulose is the product of choice here, and plain sugar is a good cheap alternative to obtaining the chemical compound. The long chain of the sugar molecules retains water in the mix by creating a gel like substance that holds moisture for longer, increasing the drying time and removing the problems caused by quick drying of hydraulic lime renders. Sugar is also useful for applying any lime based render to a high suction surface, as it stops the product being pulled into the wall too much.
There are still some plasterers out there using these tricks, and although many conservation purists are against adding anything to lime mortars and plasters in the name of tradition, it seems that admixtures for lime renders is one answer to that infamous question, “what have the Romans ever done for us?”