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  Terra Cotta Repair Once, this entry led into the old Gayoso Hotel, but until recently it was sealed over, damaged by numerous boltholes and fractures. When Peabody Place decided to renovate this as a window to the Main Street Mall, Archicast was called in to repair the ornate terra cotta.
  This meant chipping out all the plaster and mortar, cleaning the surface, epoxying the fractures, and rebuilding all the missing detail.
You can't tell where the biggest fracture was? Good! The special patching compound used was Jahn mortar. It appears off-white against the antique yellowish glaze. A short while after building it up it can be sculpted, but it's very tender. Over the next few days it hardens, if misted. You can add more at any time.
There were innumerable holes to fill, big and small, in the flat areas of terra cotta as well.

Archicast is a certified user of all the Jahn mortar products.

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Photos top and right by Phillip Parker, as well as a huge amount of computer assistance!
Photos top and right by Phillip Parker, as well as  a huge  amount of computer assistance! amount of computer assistance!
The repaired area today, a restaurant. They never had me paint the patches, as is recommended. Once you learn how the Jahn mortar works, you can build up areas quickly. But it takes sculpting ability to shape it.

Few materials cause the confusion terra cotta does.
TC is ceramic, fired clay, whose body (formula) is intended to survive harsh outdoor conditions. This generally means it has a high percentage of grog, which is pulverized old ceramic material.
Bricks are also TC. So are roof tiles, chimney liners, and the big masonry units common in older construction. We reserve the name terra cotta, though, for ornamental masonry for exterior use, typically glazed, often shiny. It can be any color, multicolored, speckled, textured or plain.
The terra cotta clay body itself is usually offwhite, yellow, tan, reddish or greyish. 
Typical terra cotta masonry units have a hollowed-out back with thick walls in a grid, and holes that penetrate these walls. You can often see the handprint of the original worker. 
People think old plaster is TC because it is yellow or pink. Plaster often turned these colors due to minerals in the mix. You can scratch plaster with steel but not TC. 
Around 1890, as skyscrapers caught on, architects accustomed to decorative design needed a material to clad structures of riveted iron beams. It had to be fireproof and they wanted a lighter weight than stone. Many terra cotta companies sprang up to fill this need. Their catalogs are marvels of showing how their myriad shapes should harmoniously fit together. 
Terra cotta typically fits into a brick facade, and the same mortar is used to glue them in place. That's what the holes in the walls are for; steel pins and cramps (Z-shapes) would be embedded in the mortar in the holes and cells. 
Terra cotta units can only be so large. One almost never sees a three foot length. So a facade made of TC shows a large number of mortar joints, more than would be necessary with stone. 
When the Memphis AIA put out a booklet called Downtown Terra Cotta, or some such, its cover showed a peacock -- made of cast concrete! I pointed this out to the author. Instead of showing appropriate embarrassment, he said "We meant decorative work in general." 

So keep this attitude in mind when even professionals get confused about terra cotta.

When the mortar is allowed to decay, water gets in. It may freeze/thaw in voids and cause failure, and it may allow the steel cramps to rust. They may break if under tension or slowly "explode" as rust is bigger than metal. Either way, the units show cracking, moving, maybe lose chunks. 
If possible, simply replacing bad mortar is a good answer. If there are fine cracks that allow water to penetrate, a special thin grout might work. If units are beyond saving, the area usually must be disassembled from above. 
Replacing a TC unit is problematic. If we make a mold directly from an old piece, the clay pressed into it shrinks, about 12%, as it is being glaze-fired. So a mold must be scaled up to do right. A mold from an old piece can produce correct-size parts in concrete, FRP (fiberglass) or Jahn mortar. Only FRP will give the glossy surface although the concretes can be coated with sealers and painted to become colored and slick. 
Concrete units will be very heavy unless they are cored (cast with a hollow core.) So an additional core mold is needed. 
FRP units usually cannot be sprayed-up with a chop gun because the mold is usually a 5-sided enclosure; you can't force air into a constricted space. So they may need to be hand-laminated. But if a large area needs to be reproduced, FRP may be preferred.
Jahn terra cotta mortar is unique in that it is engineered to have the same coefficient of expansion and the same porosity as real TC. It is weaker, and it's not easy to get the slick surface. But the recommended Silin paints give the right gloss and breathability needed. Jahn mortars are costly, so for large jobs may not be economic.

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