Yeast bank vials not freezing?

I followed FlyGuy's instructions here: https://www.homebrewtalk.com/f13/guide-making-frozen-yeast-bank-35891/ recently and whipped up 10 vials of WY1056.

Everything seemed to go well but the next day I noticed that the contents of the vials had not frozen. I had them in a smaller cooler with ice packs as suggested, so I left the door on it open thinking it might have insulated them from the cold a bit (this is a manual-defrost freezer, so no freeze/thaw temp cycles). Several days later they're still not frozen, though some have frozen bits or slush.

What's the deal here? I tried to use about 15% glycerine (which I picked up at the LHBS) but I didn't measure and it might have been more like 25%. Could that have lowered the freezing point? I should note that this freezer has frozen several gallon jugs of water solid overnight. Will the yeast be okay, if they're at a low enough temp, or am I screwed?

TIA

How do you know they are not frozen? If you are expecting ice crystals on the inside, well that's exactly what the glycerine prevents. Try filling a spare vial 1/2 full of water, and place in the center of your yeast vials. If it freezes, then all is well.

bsay said:

How do you know they are not frozen? If you are expecting ice crystals on the inside, well that's exactly what the glycerine prevents. Try filling a spare vial 1/2 full of water, and place in the center of your yeast vials. If it freezes, then all is well.

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I guess I expected the slurry to still freeze, though I don't know why... I guess I didn't see it mentioned anywhere that it wouldn't.

I'm sure that a vial with water would freeze, as everything else in there freezes fine, so I guess it's OK!

Mine freeze solid. I go 15% glycerine, measured. BTW, I have re-animated 1056 (from frozen) several times without problem.

Whether it's solid or not does not matter. What you want is for the temperature of the yeast to be about 0F. Measure the temperature of your freezer with an infrared thermometer or one of those made to go in freezers and fridges (cheap 3-4 dollars).