So today, I started a course on traditional lime mortaring. The course is being delivered by Ned Scharer from the Natural Building Centre who came to Gwrych Castle, Abergele. We started the day by learning about the lime cycle, and how lime is processed by burning limestone (calcium carbonate) in a kiln to remove carbon dioxide and water to get lime (calcium oxide), which is then turned into mortar with the addition of water to create an exothermic reaction known as slaking, as the calcium oxide tries to reabsorb the water and oxygen to leave a putty, and sand to act as an aggregate! Ned explained to us that the use of Portland Cement which has been in use since the 1830s, was used a lot to repair old buildings in the later Victorian, and later, post WWII generally, which has caused a lot of previously mortared buildings to crumble as the two substances are incompatible- lime absorbs water from the stone/brick and allows it to evaporate, whereas Portland cement is completely waterproof. So water gets under the cement and gets soaked into mortar and causes damage in frost periods! We were told that there are two types of lime mortar: traditional putty which is really nice to use! Or hydraulic lime, which is a modern invention, with a quicker setting time. In both cases, you need to use sand which has enough grit to help strengthen the mix to the desired result- the larger pieces of sand should be one third of the gap size. Once pointing has been done on a moist wall, the mixture uses the moisture to carbonate and goes hard, like stone.
So we set off to the garden wall at Gwrych which is in poor condition, and we proceeded to ‘rake’ out the old soft and rotten lime with hammers and chisels, and ended up with the following result:
Once we had that, we then mixed three different types of lime, traditional putty and two lots of hydraulic lime. We tried them all out and I can say for definite that I preferred the putty! We essentially filled the gaps, and where a stone fell out, we mortared it back in! It takes about a month for the putty to cure, and about a week for the hydraulic lime. During this time, we need to make sure the wall is wet to allow the lime to carbonate.
I’m really looking forward to tomorrow!!
Post written by Spencer Beale from Love My Wales. Special thanks to Ned Scharer from the Natural Building Centre.