Working around branding fires toughens 'em some."
"Lonesome Land"
B. M. Bower
If the water boils too low, add more hot water, never cold; the latter toughens them.
"Camp and Trail"
Stewart Edward White
2. Boiling an egg for twenty minutes hardens and toughens the white, so that it all becomes hard to dissolve or digest.
"Ontario Teachers' Manuals: Household Management"
Ministry of Education