The energy research kiln in operation
This project is sponsored by the California Department of Conservation and managed by the Center for Environmental Economic Development.
As part of this project we are testing the energy implications of using recycled glass as a raw material in brick manufacturing. Over the years there have been a number of studies of the use of glass as a raw material to save energy in ceramics kiln processes. One of the most recent can be downloaded at
One of the reasons these studies have not resulted in widespread use is that all of the previous work focused on using very finely graded glass as an additive to other ceramic raw materials. The disadvantages of using very finely graded glass include:
Uniformly crushing glass to fines is relatively expensive,
Large quantities of uniformly crushed fine glass are not currently available, and
Very fine glass hurts the workability of clay mixes.
In contrast, this study will test coarser glass, crushed to 12 mesh and finer, as a substitute for grog in brick manufacturing. 12 mesh recycled container glass is readily available as a raw material in fiberglass manufacturing. For forming bricks, the coarse glass reduces the amount of water needed and accelerates drying. This work will especially investigate the firing implications for energy use.
A special kiln was built for this study. The kiln needs to be very fast and accurate. It is always a challenge to separate the energy issues associated with the kiln itself from the firing issues associated with the material in the kiln.
For good control and monitoring, an electric kiln with quartz halogen heating elements was designed and built. Quartz halogen elements have virtually zero thermal mass and radiate instantaneously at an equivalent black body temperature approaching 3000 degrees F, peaking in the near infra-red range. They therefore do not present the complicating factors of slow heat-up and transition periods between conductive and radiation heat transfer that are associated with resistance elements.
Quartz Kiln Construction
Inside of kiln showing quartz support tubes
The inside area of the kiln is 16 inches square. The kiln is insulated with 2-inch thick Type M Board made by Thermal Ceramics Corp., contained by an expanded metal frame.
Control is provided by a Fuji Proportional-Integral-Derivative (PID) controller, with a 4-20 milliamp signal to an Omega phase-angle Silicon Controlled Rectifier (SCR). Patching into the 4-20 milliamp control signal is an Onset “Hobo” data acquisitor. Sampling rate is set at one point per second. Data is transferred to a spreadsheet, where a correlation is made between milliamp output and kiln watt input (unfortunately, SCR input signal to output power is non-linear). Total kiln wattage is from eight-1600 watt 240 volt quartz bulbs, totaling 12,800 watts at 100 percent output. Milliamp control signal is limited to 75 percent control output (95 percent power output) to assure long life for the elements.
Quartz Kiln Usage
The chart below demonstrates the speed of the kiln, as it goes from 1100 degrees F to 1800F in three minutes:
Left scale is degrees F, bottom scale is minutes
The energy analysis will be performed in several ways. Samples will be fired containing 50 percent coarse glass as the grog. After firing, the samples will be tested for absorption. Absorption will be used as the initial test for brick efficacy. The firing profile using the minimum amount of energy to make a brick with absorption less than 5 percent will be developed through repeated testing. Bricks fired using this minimum profile will then be tested for flexure strength as the final test for brick functionality.
Since the data derived from the Onset data recorder is in time/rate format, it may be possible to derive the difference between endoergic and isoergic energy requirements. The chart below illustrates the concept. The kiln was fired empty, then using the same profile (up to 1800∞F) with ten pounds of glass in molds. The energy was monitored during both firings. The upper curve represents the energy requirement during the firing with glass in the kiln. The lower curve is the empty kiln. It can be seen how the kiln needed more energy to perform the glass firing.
The violet line is energy consumption with load, blue line is energy consumption empty. Left scale is watts, bottom scale is minutes.