Historical clue formulations

• Harden and become resistant to change — 2025-12-26
• Harden into bone — 2022-10-17
• Become rigid and inflexible — 2021-04-18
• Become hardened — 2015-12-01
• Become fixed — 2015-09-23
• Become inflexible — 2014-02-11
• Turn to bone — 2013-07-21
• Harden — 2003-10-17
• Harden, in a way — 2003-10-12
• Harden — 1996-03-21