Sandstone Conservation in Chinook Country

Written by: Fraser Shaw, Heritage Conservation Advisor

Paskapoo sandstone has been maligned for poor durability, compared to Indiana limestone and sandstones from Ohio or Spain that have been used for conservation projects in Alberta. But to be fair to our homegrown sandstone, masonry in general suffers in Alberta’s climate and in the intense sunlight, drying winds and freeze-thaw cycling in areas like southern Alberta. De-icing salts used for public safety during the long winter months are the nemesis of historic masonry and will relentlessly attack sandstone, limestone and granite alike. How stone is laid in a wall, masonry mortar composition, and design details all contribute to how stone performs over time.

An imposing landmark in downtown Lethbridge, Southminster United Church is a large 1913 building (additions in 1914 and 1950) with a bold Modernist 1961 chapel. The Classical Revival original building dominates with its symmetrical front facade, prominent pediment, monumental engaged columns and exterior of buff-coloured brick with sandstone details. Interestingly, while stone decoration of the 1950 north addition superficially resembles the regional sandstone, it is actually imported Indiana limestone, a different and relatively durable material. Other notable Lethbridge buildings with this combination of local and imported stone are the Galt Museum (former Galt Hospital) and the Bowman Arts Centre (Manual Training School), both Provincial Historic Resources. Lethbridge designated Southminster United Church as a Municipal Historic Resource in 2016.

Recent stone repairs at Southminster United Church illustrate a conservation response to Alberta’s climatic challenges when using traditional materials. Stone can be quite porous and projecting stone elements are especially vulnerable to rain and snowmelt. Moisture saturates the fabric and leaches calcium carbonate binder from the stone, gradually leaving a more weakly consolidated sandy matrix. Liquid water in the porous stone matrix exerts microscopic but insidious pressure on the fabric, while ice crystals expanding in the pores effectively blow stone apart from the inside. Deteriorating stone entrance cornices at Southminster United Church are the result of decades of freeze-thaw and wetting-drying cycles.

Public safety above the main entrances made repairing the friable cornices a priority. Further degradation would lead to more extensive and costly repairs and eventually even the loss of an architectural detail. The Standards and Guidelines for the Conservation of Historic Places in Canada prefer a minimal intervention repair approach – do as little as possible but as much as necessary – using recognized conservation methods and materials. Fortunately, the cornices were still mostly intact and full replacement of the original stone would be inappropriate, unnecessary and cost-prohibitive.

A conservative repair approach of so-called indent or “Dutchman” repairs involves cutting out and replacing deteriorated stone with small pieces of matching or similar stone carved to the original cornice profile. Replicating original profiles rather than matching weathered features “resets” the clock, so to speak, and preserves vital architectural information. Such repairs are set in a compatible lime mortar and sometimes rely on stainless steel pins for extra security and holding power. However, regional sandstone quarries shut down after World War One and suitable replacement stone has been difficult to source until recently. While sandstone is again being quarried in southern Alberta, the stone currently available was not an ideal visual match for this particular project.

Rebuilding eroded stone elements with a compatible mortar, a sound and cost-effective approach for small repairs, turned out to be the best option for this project. A well-formulated mortar should be physically compatible with and visually well-matched to the historic fabric and fail sacrificially to the latter. Mortar that looks good but that is too hard or insufficiently porous can cause the repair to detach from the stone or, worse, aggravate decay of the original fabric.

At Southminster, the repair mortar consisted of high-calcium quicklime with crushed and washed sandstone aggregate recovered from the deteriorated cornice, plus a bit of sand to make up for a shortfall of recoverable crushed stone. Preparation of quicklime mortar results in a vigorous chemical reaction known as “hot mixing”. Centuries old, this craft practice was lost in the early twentieth century but is now seeing a revival in building conservation. Hot-mixed lime is the critical “secret sauce” that gives repair mortar its exceptional workability, excellent bond and porosity comparable to the original stone. Mortar requirements are case-specific and a tiny amount of added fly ash “pozzolan” enhanced durability and frost resistance, without giving excessive strength or compromising porosity and moisture transfer within the repaired stone.

With calcium carbonate as a binder and crushed stone aggregate, the repair mortar is effectively a sort of reconstituted sandstone. The excellent match of the completed repairs attests to careful mortar formulation and the contractor’s skilled hand. Discreet sheet metal cap flashings added above each cornice protect vulnerable stone surfaces from rain and snowmelt with minimal visual impact.

These repairs represent the gamut of conservation treatments in the Standards and Guidelines: preservation through the protection of sound existing material and the prevention of further degradation and loss of original stonework; the restoration of original architectural details with compatible materials; and rehabilitation through the sensitive addition of cap flashings. The use of a compatible hot-mixed lime mortar also illustrates the ongoing revival of traditional repair materials and techniques in Alberta.