Frostbite on the tips of the comb. :(

[Eggcessive]

I would not put anything on combs to prevent frostbite. Vaseline and other creams/ointments can also freeze in extreme low temps. Do you have any pictures of the frostbite on combs? Here is a good article about frostbite prevention and treatments:
https://the-chicken-chick.com/frostbit-in-backyard-chickens-causes/

 

[lysmandor]

It's artificial heat isn't it?

It is, but I don't have it in the coop now. I was mentioning it as an option.