Low-fired Brick

As proof of its durability, a key aspect of sustainability, low-fired bricks at least 4,000 years old have been discovered, thus making it one of the most durable building materials.

Low-fired brick is surprisingly lightweight, due in part to the piercings (cylindrical spaces through the center) and its porous consistency. Bucket loads of bricks can be carried or hoisted without problem. Individual bricks can be tossed up to workers with ease. 

Trapped air in the pores and piercings add some measure of insulation. The R-value could be increased with exterior insulation or by building a double wythe wall (two parallel brick walls) with air space or insulation between.

Plaster bonds exceedingly well to the grooved texture on low-fired brick, much better than typical concrete block walls. The grooves (on all four sides) are formed by the pug mill during the extrusion process.

Brick walls help minimize a building’s footprint. This is particularly important in urban areas where the cost of land is high, and in multi-unit housing and commercial construction that contain many walls.

In terms of investment value, in general, brick homes have a high perceived value and therefore a high resale value. So, this is an important consideration if you ever sell your home.

Information on low-fired brick is a little scarce, but Ecology of Building Materials, by Bjorn Berge, contains some useful details. Berge describes two types of similar brick, summarized in the following table.

 Source: Ecology of Building Materials, by Bjorn Berge 

 According to Berge, although use of low-fired brick has fallen out of favor in many areas, it was common practice on inner brick walls, areas less exposed to the weather, up to the 1950’s in Europe. He advises soaking low-fired brick before laying so they don’t absorb moisture from the mortar. Berge also recommends hydraulic lime for good bonding with low-fired brick, but to avoid pure lime products. Many builders use cement mortar.

The first step in making low-fired brick is to mix clay with rice hull ash, the same ash that’s left over from the firing process. Done correctly, adding ash can create stronger brick through a pozzolanic reaction. A mixture of 1 cubic meter clay to 2 full bags (18” x 30” polypropylene bags) of rice hull ash is typical.

The pug mill, the machine that actually forms the bricks, is often powered by a diesel engine. Pre-moistened clay is shoveled into the hopper of the pug mill, where an auger pushes it out through a metal die. The extruded clay comes out in long sections on a roller bed. Next, workers use a wire cutting device to cut the clay into brick-length pieces. Then they are loaded on a cart and stacked to dry in the sun: 4 days of sunshine or 10-15 days if the weather is less than ideal. Green bricks are stacked in rows to save space, raised about 4” above ground on concrete blocks. Plastic sheeting provides protection from rain and helps control the rate of drying. Note: low budget operations dry bricks directly on the ground and work only in the dry season to minimize expenses.

Firing process:  50,000 – 100,000 bricks are fired at one time in a large pile. The bricks are stacked with vent spaces between. Metal roofing is braced around the sides of the stack to contain it. Rice hulls are mounded on top of the pile and started with a match.  12 tons of rice hulls are used to fire 50,000 large bricks. The firing process lasts 7 days.

In addition to being a fuel source, the insulating nature of rice hulls helps retain the heat. And as the firing nears completion, the hull ash helps the stack cool gradually. After cooling, the bricks vary in color from terracotta, to tan and salmon. Black bricks are re-fired with the next batch.

From a sustainability standpoint, low-fired brick has very low energy inputs (at least in the case examined here). They are made with locally procured clay and fired with rice hulls, a by-product of growing rice. Brickyards are located near urban areas to minimize transport costs. In addition, these small businesses create a vital source of jobs at the village level. 

Drawbacks:

• At 150 kp/cm2 for solid brick, low-fired brick lacks the compressive strength of high-fired brick. A post and beam structure or a wider wall is typically required to carry roof loads.
• In addition to a bond beam along the top of a wall, a concrete bond beam at window sill height is often required to reinforce the middle of a wall.
• Low-fired brick is porous. In areas with freezing temperatures, low-fired brick can absorb moisture, freeze and break apart. However, Berge explains how northern Europeans solved this issue by using low-fired brick primarily on inner walls and plastering with hydrated lime.
• Burning rice hulls creates air pollution. However, the hulls and straw residue in fields are often burned anyway. It makes sense to utilize the hulls for something practical.
• Code compliance: Check your local codes, but typically low-fired brick is not permitted in freezing climates.

Considering the list of drawbacks above, low-fired brick does have its limitations.  However, it also has many positive attributes and a wide range of useful applications, thus providing additional options for natural builders and designers. 

References: Ecology of Building Materials, Bjorn Berge, accessed online through Google Books.