Question: A homeowner who is rehabilitating an older house wants the mason to build a new chimney top using brick selected from a big pile of salvaged brick from a demolished mill building, circa 1850. The available bricks seem to be a mixture of softer interior bricks from a 3-wythe wall and harder bricks that were originally used on the exterior. Is there some visual rule—or perhaps some simple test—to distinguish old brick that’s suitable for use from softer interior brick?
Answer: As late as the mid-nineteenth century, bricks were stacked and fired in temporary, site-built kilns, then sorted. The ones closest to the fire – “clinker” bricks – were very hard, but often distorted and sometimes fused together. At the time, they were considered ugly and mostly used for fill, but nowadays they are valued for their dark, uneven color and irregular shape. “Face” bricks were the ones located a little farther away from the fire, so they were hard and dense and retained their rectangular shape. These bricks were reserved for the exterior face on the outside walls of buildings. The bricks furthest from the fire – “fill” bricks - were slightly underfired and were separated from the face brick to be later used on the inside of exterior walls and for partitions inside buildings.
Credit: Rob Corbo
A couple of dudes are gonna have to schlep a lot of site-mixed concrete. Harsh on them.
Fill bricks are porous, so they soak up water and deteriorate rapidly when exposed to freeze/thaw cycles (see Figure 1: Caption: This garden wall was built with salvaged fill bricks in Durham, NC. In wet weather, the porous bricks absorb water; in freezing temperatures, the water turns to ice and expands, causing the face of the brick to spall.)
In general, fill brick is pinkish/orange or salmon in color and larger and less-dense than face brick. It has a softer surface than face brick; when you strike it with a hammer, you’ll hear a thud rather than the ping you would hear when hitting harder, denser face brick. But even when salvaged brick can accurately be identified, I wouldn’t recommend using it for this application. Unlike exterior walls, which often have protective eaves and rakes to protect them, chimneys—at least the portions that are above the roof line—are completely exposed to the weather.
My suggestion is to use modern face bricks for the chimney, many of which look very traditional. Unlike salvaged bricks made in the pre-industrial era, these bricks are manufactured under controlled conditions, laboratory-tested for water absorption, and much more consistent in quality. Save the old bricks for projects inside the building.
By the way, be sure to match the mortar to the bricks. As a general rule, mortar should never be stronger than the masonry units. If it is, any small stress on the structure shows up in the form of cracks and spalling in the bricks. Avoid modern masonry cement mortars, which are too hard, and use Portland cement/lime mortars (in most cases, you’ll need to go to a masonry supply house to get the hydrated lime.)To lay soft fill bricks inside, use Type O lime/Portland cement mortar. If you’re intent on using salvaged face bricks in an exterior application, use Type N lime/Portland cement mortar (see chart for proportions. CHART CAPTION: