What's the adjective for stiffening? Here's the word you're looking for.
Included below are past participle and present participle forms for the
verbs stiff and stiffen which may be used as adjectives within certain contexts.
(cooking, of whipping cream or egg whites) Beaten until so aerated that they stand up straight on their own.
(mathematics) Of an equation: for which certain numerical solving methods are numerically unstable, unless the step size is taken to be extremely small.