Always recharge the C1 Cold Cells to solid before use:
Recharge in -18C(0 F), or cooler freezer
Recharge time: 24+ hours

FAQ (Frequently Asked Questions)
What kind of freezer do I need to recharge the C1 Cold Cells?
The C1 cold cells can be recharged in any standard freezer that can achieve 0 

F(-18 C) or below cooling temperature. Always try to lay the cold cell flat 

inside the freezer until it's frozen solid (usually takes 24+ hours to get 

frozen).

Should I leave the C1 Cold Cells in freezer even if I am not going to use 

them?
Yes, it's no problem to always leave the C1 cold cells in the freezer even if 

you aren't going to use them.

Why aren't the C1 Cold Cells cold even though they've been charged for 24 

hours?
Reasons could be:
    Freezer cannot reach 0 F(-18 C) temperature performance
    Overly packed the freezer causeing no air circulation
    Freezer's door constantly opening and closing

How safe are C1 Cold Cells?
C1 Cold cells complies with USA, BS EN adn FDA standard tests which include:
BS EN- Materials and Articles in Contacts with foodstuffs
FDA Regulation on Polyethylene Test
US & EEC Toxic Elements Test
US & EEC Antimicrobial Tests
NSF Certified