Its had a pretty hard life, being used at least once a month for various things for the last 35 years (paint stripping, bending perspex, debubbling resin pours, drying paint (and heatset ink before that), weedkilling, desoldering. )
And it wasnt new when i got it.
Turns out that if you leave the planters in the conservatory overnight, the morning sun streaming through the windows is hot enough to get the water to a point where the seedlings cook.