rigid; harden; stiffen;
to become hard due to time.
You must finish before the paint can ossify. It is hard to paint when the canvas gets stiff.
ossify reminds of "iffy".
it becomes "iffy" to change it when it gets hard.
A bad computer program will ossify and become impossible to fix. Program it correctly from the start.
(c) 2016 E. Carruthers