All About Masonry: Part 2
Glazed Architectural Terra Cotta Restoration
Starting in the late 1800’s through the 1930’s, cities like New York and Chicago experienced a building boom with high rise buildings. One of the most common building materials during this time was glazed architectural terra cotta. Using it made a lot of sense; terra cotta could be repeated like a brick and made in just about any shape, color, and texture. Many buildings that appear to be carved stone are in fact glazed architectural terra cotta.
Maintaining glazed architectural terra cotta is more than an issue of preservation, it has economic repercussions as well. We frequently encounter issues caused by delayed maintenance and/or incorrect repairs, both of which can be quite expensive to fix. It is our hope that the following information will help building owners and managers avoid these pitfalls. After some general background information, we will share a case study on our restoration of the terra cotta façade at the LeVeque Tower AKA AIU Citadel.
What’s terra cotta and how is it made?
Terra cotta is a lot like brick, but a bit extra – a super-brick, maybe. Just like bricks, terra cotta is a fired clay product and its use dates back thousands of years. “Terra” means earth and “cotta” means cooked or fired, but there is more to the story.
Terra cotta has a different composition than brick, and it all starts with the type of clay used to make it. The “recipe” can also include a variety of added materials like sand or chunks of other fired clay pieces that are ground up and pulverized. If you watch someone manually pack the molds for a terra cotta unit, you will see the clay being thrown on a table repeatedly, compacting it to remove any air bubbles or irregularities before being added to the mold. This results in a very dense unit. The molds are about 8% larger than the desired final product, to account for shrinking when fired. The molds for both brick and terra cotta units dictate the shape and size of the unit, but, unlike brick, the terra cotta units are hollow on the inside. The insides of the units often have webs or internal bracing with a few holes for installing anchors to tie the units into place. The webs make the units much stronger without adding a lot of weight, much like corrugated cardboard gains strength from the internal structure. Today there is advanced machinery that can press custom shapes or extrude terra cotta just like Play-Doh, which is much easier than making it by hand. This newer approach to fabrication is most frequently seen applied as a part of rainscreen systems, which is a topic for another discussion. Side note: check out Renzo Piano’s work in Potsdamer Platz Berlin for a modern approach to using terra cotta.
Just like bricks, these terra cotta units were typically set in mortar. Behind these units, we usually we see a backup brick or hollow clay wall filled solid with grout to make a masonry mass wall.
What is Glazed Architectural Terra cotta?
Glazed architectural terra cotta is the Cadillac of clay units. There are three other types of terra cotta that we won’t be discussing here, which are brownstone, ceramic veneer, and fireproof.
The glazed architectural terra cotta unit is exactly what it sounds like: a fired clay piece with an applied glaze. Picture a teacup – the glaze is like the protective clean shell that you touch, but if you drop that cup you see there is a rougher looking interior (the bisque). The glaze can add different color and finish, and just like on the teacup, it’s waterproof (or impervious). Glazed architectural terra cotta resists water (rain and snow), and is fade-resistant too. No wonder we see advertisements that call it an “ideal” building material!
In All About Masonry: Part I, we talked about how the material properties of clay change when it’s fired. Glazed architectural terra cotta is fired to about 2,100 degrees Fahrenheit, which renders it weather-resistant, long-lasting, sustainable, and fireproof. And, it can be almost any color, texture, and shape.
So, all buildings should be made from this ‘Ideal Material’?
You know that saying: if it sounds too good to be true it probably is? In this case, it’s not too good to be true! Glazed architectural terra cotta really is great. For some projects though, it can just be kind of expensive and hard to find.
What you have to remember is that mortar is a sacrificial material (see All About Masonry: Part I), things like sealant don’t last forever either, and materials like this are only as good as what’s holding them up, which is why we see things like stainless steel clips and anchors on repair work.
The two main issues we see are 1) the materials used to support the terra cotta units, and 2) water.
Glazed architectural terra cotta was used on some of the most iconic buildings from the turn of the century, like the Flatiron, Wrigley Building, LeVeque Tower, and Terminal Tower.
When many of these buildings were constructed, there weren’t sealants like flexible silicone to close off joints and everyone relied on mortar. Long vertical joints, especially the ones at corners, need to be flexible – which mortar is not – and this is an area where we tend to see the most severe mortar deterioration.
We know that things expand when they get warmer, and the rate at which things expand depends on the material. Let’s consider a day in the life of a building: at sunrise the morning sun hits the east face of the building, starting to warm it up a bit, while the west side sits cold in the shadows. By the end of the day, the situation is reversed, and overnight everything cools off. If you add wind and rain to the mix, over the course of many years all that tiny expansion movement really adds up, creating more places for water to get in and do damage.
A little bit of water can do a lot of damage over time. Frequently, terra cotta is supported by steel angles at the edge of a floor slab, or hung from metal anchors and tied back to make things like a cornice.
What we tend to see in many buildings is that the shelf angle at each floor level, which is intended to carry the weight of the terra cotta, gets wet and oxidizes or rusts. Steel expands when it rusts and ends up exerting pressure on the materials around it. In the case of these old buildings, this expansive pressure causes damage to the terra cotta. The pressure can be so strong that we also see rusting structures damaging granite, concrete, and other types of masonry and metal panels. Today we tend to use stainless steel metal and fasteners that will not rust, but on renovation projects it’s often a different story.
The cost and time to make replacement glazed architectural terra cotta units is significant, and for sensitive historic restoration projects it is often completely warranted. We want to address the underlying causes of deterioration and damage so that when we replace glazed architectural terra cotta, we are not repeating past mistakes.
Well then, how do I fix this?
The first step is to identify the problem(s) in order to plan a proper fix. Every building has unique characteristics and conditions, but the materials and methods used to build them tend to have a lot in common. Since many of the problems we are asked to fix are similar, we are sharing a case study of the conditions and repairs at the LeVeque Tower.
Our goal at the LeVeque Tower was to create a safe, clean, and fresh street presence for a building that was more than 80 years old. Our work began in 2011 with a visual inspection. From a distance everything at the building looked fine—a little dirty, old, and tired, but nothing to worry about. The building featured a glazed architectural terra cotta in a unique oak bark texture with a mottled white and tan glaze. However, once we got into a lift and looked at it up close, we observed several issues.
We knew immediately that it was time to repoint all the mortar joints that were accessible. Fortunately, on the 16th floor there was an outdoor terrace where we were able to “mock up” our test grinding, mortar repointing, and cleaning samples.
While we were testing how to repoint, we continued to survey the building. After more investigation and research, we identified 6 issues that needed to be addressed:
6 Types of Damage
Solution: pin/anchor and patch in place; smaller hairline cracks can be surface sealed.
2a. Major Spalling
Solution: replace the unit.
2b. Minor Spalling
Solution: seal the surface.
Solution: pin/anchor in place, or replace.
Solution: no action needed.
5. Open Joints
Solution: repoint the joints.
6. Prior Repairs
Solution: clean, remove sealants, route, and reseal. Repatch damaged patches.
The first phase of the repairs involved the terra cotta at the arched windows. Scaffolding was installed for 75’ up to the 5th floor in order to provide a safe and controlled environment for the masons.
We took into consideration things like dust control, remote power, and water. We also wanted to protect the building elements already in place, such as the windows. We used a paintbrush-applied coating mask that kept mortar droppings and, later, the cleaning water and detergent off the copper windows.
Replacing the damaged terra cotta at the arched windows required significant time and expense. The masonry restoration contractors carefully measured all the pieces to be replaced and shop drawings were prepared for us to review and approve before fabrication. We sent a control sample to the manufacturer, Boston Valley Terra Cotta, and they created a master mold out of rubber and began testing different glazing mixes to match the existing building. It took months of coordination and effort before everyone was satisfied with the samples, and then it was several more months before all the replacement pieces arrived. The cost of a single replacement piece of terra cotta varied from $1,500-$5,000. The expense was due largely to the nature of the restoration: when parts are repeated and simple they cost less, but custom building a mold for a single shape to be hand-packed and fired only one time is a large investment. We prioritized the repair work accordingly, trying to repair as much terra cotta as we could on-site rather than replacing it. When the scaffolding finally came down, the result was truly fantastic.
We continued to test and diagnose other areas of the building using our grading key and a technique called sounding, where a special mallet is used to tap the building. When conditions are good and the material is solid, there is a dull thud sound, but if a terra cotta piece is broken or there is masonry backup missing, it sounds hollow like a drum.
After finishing the first phase of restoration, we turned our attention skyward to the remaining 480 feet of the tower. Due to the tapered shape of the building, finding a way to observe and catalog the scope of repair was an immense undertaking. We reached out to Vertical Access to solve the problem; these folks are a perfect mix of fearless athletes, technicians, and engineers.
The team rappelled from the top of the building to the bottom with laptops and tools to measure and record the six conditions we identified.
The final report was used to prioritize ongoing repairs that continued around the building.
The work to restore the LeVeque Tower began with research. Even though glazed architectural terra cotta is a prevalent material in our cities, finding qualified folks that work with and fabricate the material can be difficult and expensive. We have seen fiberglass and cast stone products used as appropriate substitutes, but determining when a substitute is appropriate is another matter. At the LeVeque Tower, it was the Historic Preservation Tax Credits at the state and federal level were what really enabled all the restoration work. Our goal was to invest in quality repairs today that will buy another 80+ years of service, enabling this iconic structure to remain a part of our city for generations.
The National Park Service is a great resource for preserving historic properties with 50 Preservation Briefs that provide guidance on the appropriate treatment of building materials and features: https://www.nps.gov/tps/how-to-preserve/briefs.htm