Wall Plaque

what makes the plaques in wall mount gas heater go bad?

for some reason my wall mount gas heater has a horried odor, the plaques are blue instead of a reddish glow. Do I need new plaques .

They can get dirty. I took my heater to a air compressor and sprayed them. two of three are now ok. I asked the guys at work who replaced several today how they worked and all they had to say is they get dirty and if there is some obstruction it prevents them working right. also some of ours had gotten broken and had to be replaced.

Hellraiser masks, and a pinhead wall plaque