One of those words missing from Webster 1913, congeal is a verb which is used to describe the change from a soft or fluid state to a hard or rigid state. In cooking, this often occurs through cooling or freezing. For example, to remove fat from stock or soup, refrigerate it; the fat will rise to the top and congeal, after which it can be easily spooned off and discarded.
Congealing can be less useful; think old milk that has soured and coagulateed. Ideas and opinions too can congeal into dogma, becoming fixed and unchangable.