Instructions
1. Read this NYTimes recipe
2. Be delighted that:
a) all the ingredients can be found in Uganda (unlike this heartbreaker), and
b) it calls for a stove, not an oven (good for those "without power" nights)
3. Ignore words like flameproof casserole dish and flame tamer on the stovetop
4. Proceed fearlessly
5. Smell the burn
6. Hear the POP!!!
7. Find pieces of exploded casserole dish and eggplant all over the stove and kitchen floor
8. Scrape, salvage, serve what's left. Piles of fresh tomato, avocado, cilantro, and homemade yogurt do wonders to cover up the sorry-looking main dish
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| Flame tamer - maybe not so optional? |
Hi Mara; read your blog on discovery of thermal expansion effects in brittle materials, part 2 (remember your oven stone experience in PIttsburgh?).
ReplyDeleteIf you look at nonmetal allowable strain levels [calculated from the max. allowable stress (the tensile strength) combined with the typical stress/strain value for the materials (the Young's Modulus)] where strain is the fractional change in the material's length in any direction, you find the following typical numbers, with Zirconia ceramic being used below ...
Compressive strength 2000 MPa
Tensile strength 300 MPa (note that like concrete it can be compressed hugely but stretched only a little)
Young's Modulus/Tensile Modulus = (stress/strain) = 200 GPa
So strain (max) = Tensile stress (max) / Young's Modulus = 300 / 200,000 = 1/1000, roughly.
Thus to fracture all you need is for one side of the material to expand fractionally 1/1000th more than the other side and you have a break. With an expansion coefficient of say 10^-5 per deg C, you need 100 deg C difference between top and bottom and OPOPOPOP what have I done! All depends on the localisation of heat input and consequent temperature differences.
Compare the situation for metals; say 304 stainless steel;
Tensile strength 300 MPa before plastic strain starts (ie, irreversible loss of shape)
Tensile strength 600 MPa ultimate
Young's Modulus/Tensile Modulus = (stress/strain) = 200 GPa
So strain (max) = Tensile stress (max) / Young's Modulus = 600 / 200,000 = 3/1000, roughly; 3 times the allowable strain in above ceramic.
Combined with the maybe 5 times thinner material for stainless steel pots and almost 10 times higher thermal conductivity of the basic material than glass or ceramic, you end up with needing about 50 x 3 = 150 times higher thermal loads to fracture the metal (though it can be done if melting or buckling distortion does not happen first).
Does this help, dearie? Chris.