Flash photography used to be pretty wild [video]

The video shows that inside the flashbulbs you'll see a small blue dot. This was not discussed in the video nor was it mentioned in the comments—at least the ones I had time to read.

This dot is an important indicator in that it should always be blue, if it had turned pink then the bulb may not fire because the bulb's hermetic seal had been broken and air had gotten in (glass cracked, bad metal/glass seal on the firing electrodes). The ingress of air would dilute the oxygen atmosphere and likely oxidize the metal zirconium thread due to the presence of water vapor and the bulb wouldn't ignite or its light output would be suboptimal.

The blue dot would turn pink in the presence of air due to traces of water vapor in the air. It consisted of cobalt chloride which is blue in its dry state but turns pink when damp.

Some may recall cobalt chloride was used on ornamental weather indicators. As well as little figures of a man and woman popping in and out of a house to indicate dry and wet weather a large round dot of blotting paper soaked in cobalt chloride was used to do the same. In dry weather the dot would turn blue and wet it would turn pink.

Incidentally, when I was a naïve kid I asked my local pharmacist if he had any cobalt chloride so I could make my own indicators and [surprisingly] he actually had some and he put about 20gms in a small glass bottle and sold it to me for a few pence. He went to considerable effort to tell me that cobalt chloride was poisonous and he labeled the bottle with its contents and a big red poison label.

Oh how times have changed.

Edit: cobalt chloride also used to be widely used to indicate moisture in desiccants such as silica gel. Small amounts of blue cobalt chloride crystals would be added to dry gel. If they turned pink then the gel had become hydrated and needed to be replaced. Recently, it's been removed due to health concerns.

And now I know what was in the stem that set them off. Finally.