The south facing wall of Morningstar Mill was repointed in September using a traditional hot-mixed lime mortar. Lime mortars have been used in buildings for thousands of years and continue to be used for the repair and maintenance of historic structures built before the beginning of 20th century.
Our process began by having a sample of the mortar used in the original construction of the mill examined by faculty from the Willowbank School of Restoration Arts. A closely matching mortar was reproduced by simply combining lime with sand and water and a little clay. While this sounds easy enough, there is a lot of chemistry and hands-on masonry experience behind the mix and its application.
Lime is produced when limestone is fired in a kiln at a temperature of around 850 degrees Celsius for about 8 hours. This drives off the carbon dioxide held within the limestone and produces a highly reactive solid known as ‘quick lime’. When water is added to the quick lime, it generates heat and produces lime which is the binding agent in the mortar mix.
The sand used in our mix is orangey in colour and was purchased from a quarry in Pelham. The sand, quick lime, water and clay were measured out in a pail and then mixed together on a sheet of plywood. As the water was carefully added to the quick lime, and it immediately began to pop and give off steam. During the process, the mix became so hot that it burned the plywood! The mix was pounded with the end of a 2×4 in order to aid in the binding and mortar mixing process, and then turned over with a shovel many times. After a few minutes, the mix became quite malleable and plastic.
Before the new mortar was applied to the wall, the old mortar was removed with chisels and grinders, and the remaining cavities were brushed out and misted with water. The grinding process took about one day, and the mortar work took about two days.
While the new lime mortar work is now covered by the Morningstar Mill sign, examples of the original lime mortar can still be seen on the mill. It is easy to recognize – just look out for the gritty orange-coloured mortar with the small white lime clumps!