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The two graphs below are the "true" strain in glass after the calibration of subtracting the ceramic strain from the glass strain. The plots show a fairly linear relationship between the strain and temperature of glass. Also, the difference in the slope between the two perpendicular directions (0 and 1) proves that this glass is not an isotropic material---its strain in the 0 direction is larger than that of 1 direction.
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Future Experiment
If none of the hypotheses proposed in the "Revisiting the Purposes" are the actual causes of the glass breaking of the solar oven, there is another direction to take for investigating the glass breaking issue. It is possible that the uneven heat distribution and the high rate of temperature change in glass may be connected to the breaking issue. It is known that the temperature inside the oven cannot go up anywhere near to the melting point of glass. Technically, as long as the heat distribution is more or less uniform around the glass and the rate of temperature change is moderate, the glass should not break. The glass would just wait until the glass reaches the melting point without ever breaking. This is similar to the glass breaking phenomenon when the extremely hot water is poured onto a glass surface. Glass is a poor conductor; this means when one side of the glass is significantly higher than the other side of the glass, this will cause high stress distribution within the glass and eventually the glass would break. In the solar oven case, the inner glass is placed in between the inside of the oven and the outer glass. The glass is insulated by the silicone caulking around the edges. The diagram below explains a possible way the inner glass could break.
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