Chimney Brick: Types, Ages and Salvaged Brick

One material, a wide range of quality

Brick is clay or shale shaped into units and fired in a kiln. The firing turns soft clay into a hard ceramic, and how completely that happens decides much of how the brick behaves for the rest of its life. The Brick Industry Association's note on manufacturing explains that, for the same raw material and method, darker colors go with higher firing temperatures, along with lower absorption and higher compressive strength. It also points out that absolute uniformity is impossible because temperatures vary inside a kiln.

For a chimney, absorption is the property that matters most day to day. A brick that soaks up more water holds more of it when the temperature drops, and the freeze-thaw and salt damage described in why chimney brick flakes both need that water to work.

Three ways brick has been shaped

BIA describes three forming methods, and each leaves its mark on the finished brick:

• Soft-mud, or molded. Wet clay, 20 to 30 percent water, is pressed into molds lubricated with sand or water, giving sand-struck or water-struck brick. All molding was once done by hand. Molded brick often has softer edges and a creased or sandy face, and BIA notes it cannot be made to the tightest dimensional tolerances. Some molded brick has a recess, called a frog, in one bed face.
• Dry-press. Clay with very little water, up to 10 percent, is pressed into steel molds under high pressure.
• Stiff-mud, or extruded. Clay with 10 to 15 percent water is forced through a die as a continuous column and cut into units with wires. BIA reports that about 90 percent of US brick is made this way today. Extruded brick often has holes, called cores, through it, and its faces may be textured by cutting, scratching, rolling or brushing.

BIA dates the arrival of brick-making machines to the latter part of the 19th century. That is a broad marker, not a way to date a particular chimney: hand-molded and machine-made brick overlapped for years, and molded brick is still produced for its look.

Clues you can read from the brick itself

These observations help describe a chimney. They do not tell you whether it is sound. Condition comes from looking at the whole structure, including the crown, the joints and the flue.

What SW, MW and the other labels mean

New brick in the US is sold to ASTM specifications, and BIA's note on brick classification summarizes the ones that matter for a house:

• Facing brick (ASTM C216) comes in durability grades SW and MW. S stands for severe weathering and M for moderate. Appearance types FBS, FBX and FBA describe how tightly size, chips and color range are controlled.
• Building brick (ASTM C62) is made for brickwork where appearance is not a requirement, and adds a third grade, NW, for negligible weathering.
• Solid versus hollow. A "solid" brick under these standards may still have cores, up to 25 percent of its area. Above that it is hollow brick, covered by a separate specification.
• Firebox brick for residential fireplaces has its own specification, ASTM C1261, and is a different product from the brick on the outside of the chimney. See firebrick, refractory mortar and firebox panels.

BIA's maintenance note says chimneys, along with sills, parapets and copings, take harsher weather than ordinary walls and can need repair sooner. That is why, when brick is bought for chimney work, its durability grade is worth asking about alongside its color.

Why an old chimney can hold bricks of very different strength

BIA's note on salvaged brick gives some history. Early 20th century kilns fired unevenly. Brick from the hottest zones came out hard; brick from cooler zones was under-burned, more porous, slightly larger and lighter in color, and became known as salmon brick for its pinkish-orange shade. Masons building thick load-bearing walls sorted them, putting hard units on the weather face and salmon brick in interior wythes where the elements would not reach it.

Two practical points follow for chimneys. First, a chimney that has been standing for a century may contain softer brick in places that were never meant to get wet, and those places start getting wet when a crown cracks or a flue leaks. Second, once a structure is taken down, BIA says it is virtually impossible to tell the durable units from the non-durable ones, because the color cues the original masons used fade with exposure and contact with mortar.

Salvaged brick: the look and the risks

Reclaimed brick appeals for good reasons. It can match an old house, and BIA itself acknowledges that it may satisfy the wish for a rugged, colorful surface. Its technical note is still one of the more cautious documents the association publishes. It makes these points:

• Mixed durability. Hard and soft brick end up mixed during demolition, so one pile holds two very different grades, and masonry built from salvaged brick is quite likely to spall, flake, pit and crack from freezing and thawing.
• Weak bond. Old mortar and absorbed cement particles clog the brick's pores. Tests cited in the note found mortar bond to reclaimed brick to be less than half of what the same mortar achieves with new brick, and the bond between new and old mortar is not very strong.
• Testing. The note quotes building codes of its time that required reclaimed units to be tested before structural use, and says compliance cannot be assumed without testing.
• Chimney brick in particular. BIA states that masonry units salvaged from chimneys are not recommended, because they may contain oils or tarry material.

BIA's suggested alternative is new brick blended from several colors, which many manufacturers offer to give a rustic look without the durability questions. The salvaged brick note dates from 1988 and is still published on BIA's website.

Matching brick when part of a chimney is repaired

Most chimney masonry work replaces some brick and keeps the rest, so the new units have to live beside the old ones. BIA's maintenance note says replacement brick must match the existing brick in color, texture and size and also in physical properties, and recommends mock-ups or sample panels when choosing. The National Park Service's guidance on repointing makes a related point about appearance: replacement units should blend with the full range of colors in the wall, not with one single brick.

Physical match matters because a much harder, less absorbent brick set among soft ones changes how water moves through the wall. The mortar has to fit as well. NPS advises repointing with mortar softer or more permeable than the brick it sits between, a point covered in more depth in lime, portland and refractory mortar.

On our assessments, the exterior masonry is checked from the roof when that is safe, with photos of the brick, joints and crown in the findings. Those photos show whether damage is limited to a few units or spread through the stack, which is the first step in choosing between masonry repair and rebuilding.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 9: Manufacturing of Brick (PDF)
2. Brick Industry Association, Technical Note 9A: Specifications for and Classification of Brick (PDF)
3. Brick Industry Association, Technical Note 15: Salvaged Brick (PDF)
4. Brick Industry Association, Technical Note 46: Maintenance of Brick Masonry (PDF)
5. National Park Service, Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.