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Conservation Notes: Plastering The Grange

February 18th, 2015

By Claire Molgat-Laurin, AGO Conservation intern

Conserving the AGO’s largest artifact

The AGO’s largest artifact isn’t a monumental painting, or even a sculpture. Think even bigger.

Built in 1817, The Grange was the first building to serve as the home of the AGO, then known as the Art Museum of Toronto, and it is the oldest remaining brick house in Toronto — an important piece of history both for Toronto and for the AGO.

But over the course of the past few years, Jennifer Rieger, the historic site coordinator for The Grange, noticed that paint on one of the basement scullery walls was becoming powdery and blistered. This was a worrying symptom, one that indicated water was trapped in the walls and was damaging the plaster underneath the paint.

Henry Bowyer Lane British, 1817 - 1878, The Grange, c. 1840, overall: 28.6 x 44.1 cm (11 1/4 x 17 3/8 in.), watercolour on paper, Gift of Mrs. Seawell Emerson, Marblehead, Massachusetts, 1972, transferred from the Grange, 2008. © Art Gallery of Ontario

Henry Bowyer Lane
British, 1817 – 1878, The Grange, c. 1840, overall: 28.6 x 44.1 cm (11 1/4 x 17 3/8 in.), watercolour on paper,
Gift of Mrs. Seawell Emerson, Marblehead, Massachusetts, 1972, transferred from the Grange, 2008. © Art Gallery of Ontario.

A house like The Grange is like a living organism, with each part of its construction supported by another. If one component of the house is out of balance, it can affect the rest of the building too. The plaster covering the walls of The Grange should function as its skin, protecting the walls of the house and wicking dampness away from them.

Finding the root of this problem took some serious digging through the archives of the AGO to find records of previous repairs, sending samples of the materials in the wall to be analyzed by conservation scientists and consulting with a preservation architect to assess the exterior of the house. As we learned more about older repairs and the materials used, the situation started becoming clear: one of the problems with this wall originated during a restoration treatment that undertaken in the 1970s.

One of the most important principles in conserving artifacts and art is to use compatible materials that will react to the surrounding environment in the same way as the original components. The old restoration of the plaster in The Grange had been executed with materials that were thought to be compatible at the time. However, in more recent years, it has now been found that these newer, more mainstream materials definitely don’t combine well with traditional materials like those used in the original construction of the house.

Exposed structural wall in The Grange.

Exposed structural wall in The Grange.

The Portland cement and bonding agent used underneath the repaired plaster didn’t react to moisture in the same way as the original lime plaster layers. Instead of allowing moisture to dissipate through its surface, the bonding agent blocked the movement of moisture, causing the plaster to deteriorate and any attempted repairs to fail. The analysis of the materials also suggested that the wall was retaining moisture because the plaster hadn’t been left to cure long enough before the paint was applied, which didn’t allow the plaster to breathe.

To repair the plaster, we called in the services of specialists in heritage plastering — Ben Scott and James Sloan from the Lime Plaster Company. Both have a lot of experience working with historic buildings and know lime plaster inside and out.

According to Scott, working with historic houses like The Grange is complicated, but there’s one simple approach: every historic building is going to be different, and you can’t treat them all the same way.

Most of the time, going back to tried-and-true traditional methods is best. This is a new repair, but the materials have been used in construction all over the world for thousands of years: slaked limes, non-hydraulic or hydraulic limes, animal hair and assorted natural aggregates. The plaster used by Scott and Sloan is their own mix of aged lime putty plaster, unadulterated by any other materials that could be in ready-made mixes.

A closer look: Exposed structural wall in The Grange.

A closer look: Exposed structural wall in The Grange.

It’s a process that takes skill and experience and, most of all, time. Before starting the new treatment, they completely removed the old repairs, paring the wall down to its granite rocks to get a good surface for the plaster. They then applied several layers of plaster over the course of several weeks, starting with a strong “pricking-up” coat that prepared the surface for other layers, then gradually building up to the smooth outer finishing coat. Finally, the plaster needed time to cure as the lime react with carbon dioxide in the air and hardens, forming a strong bond.

The new lime plaster will allow The Grange’s wall to breathe again by effectively drawing moisture away from the walls to help protect the house. A well-executed plastering job like this will last a lifetime, like the original walls of The Grange.

“They don’t make them like this anymore,” Scott said, patting the wall of The Grange.

All the more reason to keep it breathing.

Curious about Conservation?
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