Stonemason Mark Whyte was recently onsite at the Sarjeant Gallery Te Whare o Rehua Whanganui in Queen's Park, assessing the condition of the historic building's limestone exterior.
The 100-year-old Sarjeant Gallery is closed for earthquake strengthening and construction of a new wing – Pataka o Sir Archie John Te Atawhai Taiaroa. As a heritage restoration expert specialising in the conservation and restoration of stone and brick buildings, monuments and memorials, Whyte has also worked on parts of the Christchurch Cathedral, which collapsed in the 2011 earthquakes.
Whyte was in Whanganui documenting the condition of the Oamaru sedimentary limestone that gallery is built with, determining causes of any deterioration and recommending appropriate repairs to be done.
Although there is damage to be repaired and ongoing maintenance to be done, he concluded the Sarjeant is one of the finest examples of this kind of stonework he has seen in New Zealand.
"P. Graham and Son were really good stonemasons, the horizontal lines on this structure are perfectly level. All the pointing gaps are even. The perps [the vertical gaps between the blocks] and the bedding joints [the horizontal layer on which the blocks are laid] are all even," he said.
It's not well known that pointing work between the stones is considered to be a temporary, sacrificial element.
It is not intended to last indefinitely and requires renewal and maintenance throughout the life of the building.
Typically, on a building like the Sarjeant, the pointing should be reviewed and addressed as necessary every 10–15 years.
It's also essential that the correct chemicals are used in the mix of pointing so the stone doesn't deteriorate.
Whyte's report reveals that the most serious surface damage is from pigeons' acidic excrement eating into the limestone, and from moss and lichen – their roots get in, then moisture, which freezes in the cold, expands and cracks the stone surface.
Whyte also said that much of the structural damage that can be seen at the corners of the building was not from earthquakes, rather from rust expansion of the internal cast iron downpipes.
He recommends they are replaced with plastic downpipes as part of the restoration.