I suppose if the glass is not made properly, little gas bubbles may be in there too tiny to see.

You know I am reminded of a show I saw on one of the science channels about building tall skyscrapers.

It went into the glass making in detail. Did you know, according to this show, that when it is vertical..as in a window, glass actually "flows" over time to the bottom of the pane, and becomes thicker there? When this happens the pane may crack and break all by itself!

So what we see as a solid, is actually a very slow moving liquid!

I don't really favor candles much. I am a bit afraid of them. And given our cats that live with us... it is too easy to have an unexpected accident.

I am reminded of this funny story to illustrate that. UpNorth, we have these black flies that come out in droves before a rain to bite you. They come down our chimney too, after a fire to get to the warmth of the house. One year I bought a sticky tube, and hid it on the sill behind our dining table by the window where these pesky flies tend to congregate trying to get back outside. Well one night in the dark Tippy managed to slink back there and get her hind end stuck to the tube (it is like the tube in a roll of paper towels). She burst out screaming and knocked over a kerosene lamp which spilled all over...and luckily did not catch fire. It happened oh sooooo fast..and without electricity it was hard to see what she was so upset about. We followed her up the loft where she hid, carrying on something awful, and I urged her out of from under the bed, holding my flashlight. Then I saw what the problem was! Getting it off her was a huge problem, and unsticking her, with mineral oil, then got all over her and she ended up vomiting the next day several times trying to get the oil off herself.

So ever since then I don't have candles. Hubby likes them, but I think the risk with having cats, is just too great!

You bring up a good point however. I think most of heat is going up in the short candles. But I recall some tall ones in the past, but our few ones then never broke that I recall.