(...)
You are right of course.
I retract my statement about porcelain because it has only 'moderate' thermal shock resistance in relation to the other varieties of vitreous whiteware which range from 'good' to 'excellent' in thermal shock handling capability.
Apparently sparkplugs are weenies WRT thermal shock. :)
Or so says The CRC handbook of mechanical engineering (Second Edition, bottom of page 12-97) By Frank Kreith, D. Yogi Goswami
--Winston :)