Petrograph is Not a Dinosaur

by Christopher Swan, AIA

I think that it is important to discuss the need and process for taking care of the mortar joints in a historic masonry building. If you read the paragraph below, then you have to promise me that you’ll read the subsequent paragraphs as well.

The best thing that an owner of a limestone building – or any masonry building for that matter – can do is regularly keep up with the mortar joints. When you cut out the old, deteriorated mortar joints and put in new mortar, this is known as pointing, repointing or tuckpointing. It is an age old process that many do, but only a few do well. You can expect to repoint the building’s mortar joints every 50-75 years. In the interim, though, it is critical that the condition of the mortar joints be regularly inspected and repaired to prevent water infiltration. When there is a loose or non-existent mortar joint, I recommend immediately filling those joints with new mortar.

Now stop. Before someone breaks out a trowel and starts mixing mortar, we have to do our homework. The very first thing that needs to happen is that mortar is tested for its composition. You can have a wet-chemical or instrumental test that analyzes the mortar, but the most common is a microscope instrumental test called a petrographic test. This test will give you the materials used in the mortar and the proportions of the mortar mix. This is not something that just anyone can do. It is critical that this be performed by a qualified testing agency that has both the tools and the knowledge to provide the right information. This typically costs between $1,500 to $2,500 for the test and a multi-page report. I know this seems like a lot, but it will save thousands of dollars in costly repairs later. Also, once you have the composition and mix design, you can use it over and over, so it is a great investment regardless.

The reason for this testing is two-fold. First, to be historically accurate, it is crucial for matching the color, texture, and authenticity of the mortar. The second is the most important – the hardness of the mortar will be appropriate for the application. In limestone buildings (and most masonry applications) the softer the mortar that is used, the better it is for the masonry. This allows the building’s stone to expand and contract with changing temperatures and the mortar joints will take the brunt of the movement.

You usually don’t think about your building moving, but it does every day. A common misconception is to use a really hard mortar that stand up to the weather. Please resist this temptation. If you use too hard of a mortar, the masonry will begin to spall and crack. This deterioration of the stone happens quickly and in many cases, the damage is irreversible.

If you need help finding a testing agency, give me a call and I can help locate one that makes sense for your building.