Making a quill and one of the methods involves boiling in alum and salt. Looking for a chemical explanation of this might be for.

According to wikipedia the alum is made of "The specific compound is the hydrated potassium aluminium sulfate (potassium alum) with the formula KAl(SO4)2·12H2O."

As for the feathers, "The β-keratins in feathers are composed of protein strands hydrogen-bonded into β-pleated sheets, which are then further twisted and crosslinked by disulfide bridges into structures"

The feather turns translucent when boiled in this alum and salt.