Cadmium

I was working out of my office yesterday, in a building that is basically a line of industrial workshops. I was doing some necessary but messy work and when I was done I swept out my workspace. The floor was dusty and by sweeping I put that dust into the air and no doubt breathed some of it in. It turns out that in one of the workshops at the end of the building is contaminated with cadmium. I doubt that sweeping the floor at the other end of the building exposed me to much, but I have worked with cadmium paints in the past so I did some looking into its toxicity.

Discovered in the 1800's, cadmium is a metal that has a bluish-silver look to it. It is soft and easily manipulated and can be a byproduct of zinc production. Since our bodies have no use for this metal, it is toxic to us even at low concentrations. To me the scariest part is that cadmium bioaccumulates – there has been reports of cadmium poisoning from crops downstream of mines. Ingesting it over a long period of time can result in kidney disease and various types of cancer – but inhaling cadmium fumes can be fatal relatively quickly.

Even with its scary side, cadmium is interesting stuff. I first encountered cadmium as a pigment component; it makes brightly coloured yellows, oranges and reds that last for centuries without losing their original bright hue. These shades replaced older, not light-fast pigments such as vermilion (mercury sulfide). Cadmium has also been used as a corrosion-resistant coating for steel and as a stabilizing compound in plastics.

Because of its toxicity, cadmium use is in decline. There are now non-toxic replacements for the cadmium pigments, it has been phased out of plastics and many of its other uses. Currently, most of the cadmium produced goes into nickel-cadmium batteries, which leaves a worry of how these batteries are disposed of.

Playing With Fire

I remember making candles when I was a kid. We filled empty and cleaned food cans with paraffin wax – the stuff that comes as a opaque block about the size of a deck of cards. The cans were then put into a bath of boiling water until all the wax melted. A few crayon stubs were added to each can, creating an array of colours. I tied a thick piece of cotton string, destined to be the wick, around a pencil for easy dipping. Next, I started dipping. With each dip another layer of wax clung to what was already there, increasing the diameter of the candle-to-be. I rotated through the colours, creating what must have been gaudy candles. When the candle was thick enough to stand on its own, the fun part began: we could light them.

A flaming match held to the exposed end of the wick has enough heat to vaporise wax within the wick and react with the oxygen in the air. Within moments a teardrop-shaped yellow flame flickers to life. The heat from the candle's flame melts the wax, and the melted wax is drawn up by the wick, sustaining the flame. At its hottest, a candle's flame can reach 1400 degrees Celsius. What is actually happening? Heat vaporizes the wax creating a gaseous cloud where the combustion takes place. Combustion is a series of chemical changes that converts molecules into new combinations – however this process isn't totally efficient resulting in the production of heat and light. Light, along with its cousin heat, are part of the electromagnetic spectrum and signify the release of excess energy.

Candles used to be one of the main ways to create artificial light. However, compared to an incandescent light bulb, a candle produces 100 time less light, which is probably why candles are now mostly used to set moods, conduct rituals and provide light in power outages. I don't often light candles, after all they are one of the leading causes of residential fires and they put soot and chemicals into the air I breathe. Some candle shops are so over-scented I can't even stand being in them: I can't imagine what my house would smell like if I burned their candles! But, when I do have a reason to light a candle, I enjoy watching the flickering flame – I find something about it quite mesmerizing.

One of the discoveries from experiments conduced in space is the importance gravity has in the formation of a flame. Here, in my mundane earth existence, when I light a candle the hot gases formed are less dense than the air around them, and so they rise in a process of natural convection into the familiar teardrop shape. This natural convection hinders complete combustion, so soot forms which makes the flame yellow. Out in my funky futuristic spaceship, where there would be no gravity (unlike the spaceships on TV), natural convection wouldn't occur, and I would get a perfectly spherical flame. In space, my flame would require ventilation or it would smother itself. Its temperature would be evenly distributed and combustion would be complete, so soot would not form. The flame would be bluer and more efficient.

Another effect of gravity on a candle's flame is the flickering. The frequency squared of a flame's flickering is proportional to the force of gravity over the diameter of the candle. Meaning that a candle with a smaller diameter would flicker at a faster rate than one with a larger diameter. So a candle on another planet (with different gravity) would flicker at a different rate than the same candle on earth. A candle on my spaceship wouldn't flicker at all (I would have to be mesmerized by its pretty spherical blueness instead).