Aging gracefully, accepting discolourations

Published on 03 November 2021

coronation decorations on HQ when it was post office.jpg

The nineteenth century builders and architects who settled in the Waitaki found very little timber but an abundance of excellent building stone and they made good use of it. Skilled stonemasons constructed the many ornate stone buildings – giving Waitaki, and particularly Oamaru, a unique architectural identity: an outstandingly significant heritage townscape of commercial and civic buildings. These buildings need care and attention to look their best and to preserve them – cleaning the stone is one aspect of caring for your Oamaru stone building.

Building conservator Robin Miller of Origin Consultants, and stonemasons Dooley’s Masonry have been working together to inspect Oamaru’s former Chief Post Office. In 1883, Oamaru architectural partnership Forrester and Lemon designed in French style what is now Council HQ. This grand building is a testament to the importance of communication and postal services in nineteenth century New Zealand, and ‘the most splendid and ambitious monument to civic pride built in Oamaru’.  

Robin Miller was in Oamaru last week. “We took a huge amount of information looking at the building from a crane platform and scissor lift. It’s too early to try and summarise what we found as our analysis will take a while to do.  We’ll be working on preparing an external condition report for the stone walls and clocktower over the next few weeks.” To understand what’s going on with a building and the works that need done, Robin prepares a cyclical maintenance plan – catching problems early saves money and means less invasive works are needed. Cleaning is part of a cyclical maintenance plan.

Robin tells us some of the things to consider when embarking on the cleaning of stonemasonry – inappropriate cleaning is probably the most common cause of damage to masonry buildings. Professional advice is important: a building conservation professional or stonemason who has experience with Oamaru stone can help. The skill and experience of the person doing the cleaning is key.

When considering cleaning, Robin first assesses the building and identify the causes of any staining and discolouration. Once he understands what is going on, he chooses the gentlest cleaning method that is likely to be effective – a combination of methods may be necessary.

Robin may trial different methods to see which work best, starting with the gentlest. Naturally, a strong motivation for cleaning is aesthetic reasons – a desire to have the building looking as good or as new as possible. However, Robin says, an overly cleaned building is a building that will be susceptible to accelerated decay and soiling. Just like us, a building can age gracefully with an acceptance of subtle discoloration and visual imperfections.

Heavy cleaning can remove the protective surface of the stone leaving it vulnerable to water ingress, pollutants and salts. Robin also considers what previous work has been done on a building. Historic application of sealants or water repellents will change how the stone responds to cleaning.  There’s a lot to understand when you consider cleaning your building!

Next time you walk past Council HQ, look up and do a bit of amateur sleuthing. See if you can see dark patches and weathered stone – all things Robin will be considering in his report.

We will keep you up to date as the work progresses. And if you own an Oamaru stone building and would like to have a chat about maintenance, our Heritage Advisor Heather Bauchop is here to help.