One day a few years ago, Callan Bentley posted this on Twitter:
I had no earthly idea what Liesegang banding was, but if Callan says it's the best, you know it's something good. So I clicked through. Do it. Go on.
Amazing, isn't it just? Made little lights go flash for me, because it turns out I've spent a lot of time surrounded by Liesegang banding. It's all over the sandstones in northern Arizona, and we used to sell bits of it in our bookstore, coasters and bookends and such. It's marketed as "picture sandstone." The patterns are gorgeous. When I saw stacks of coasters at the rock shop near Gingko Petrified Forest State Park, I had an acute onset of terminal nostalgia and bought some. Lovely!
And now I know what caused those incredible patterns. Well, sort of. We haven't quite figured out the processes that cause Liesegang banding. And by "we," I don't mean me and my cat, although the two of us don't quite understand it. No, the whole scientific community is still scratching its collective head over the particulars. But we've got some broad understanding. It's not a complete mystery, just one of those mysteries that keep scientists happy and busy.
In the meantime, we get to look at the pretty results. Imagine my delight when, in a Google search for Liesegang banding, I got led to none other than Brian Romans's very own blog, and this gorgeous field photo. You really must click to see.
I spent my teen years galloping over rocks very like those, magnificent Jurassic sandstones formed from ancient dunes. I thought the pretty stripey colors, all of the yellows and mauves and reds and deep dark browns, happened at the beginning. But no! That came later. First, you had your dunes, then you had your sandstone, and then along came groundwater, dissolving all those lovely iron-rich minerals like hematite, and precipitating it out.
Then, after a great many years and a general dry-out, you get wildly-patterned formations like the above, and some bugger comes along to quarry them for things like this:
It's not just sandstone that gets your Liesegang bands. It can happen to tuff, too, and even man-made things like lime mortar (but only the Roman recipe stuff that's aged 14 years and similar). All that's required is something suitably porous. The phenomenon was first noticed in blotting paper by a man named Frederic Ferdinand Runge in 1855 - he was so taken by his "self-painting pictures" that he wrote the book on them. But he failed to take the world by storm, so it was left to Raphael Liesegang, futzing around with photographic chemicals forty-one years later, to rediscover them. In one of those wonderful scientific accidents that leads to discovery, he dropped a crystal of silver nitrate onto his gels and saw concentric rings form. Instead of throwing the mess out, he wrote papers about it.
In nature, things aren't so neat as his concentric rings. Liesegang bands appear as, of course, bands, but also rings, spirals, and spheres, oriented in various directions, with younger sets cutting across and sometimes dissolving older ones.
One of the most fascinating things about these bands is that they're formed by a self-organizing process: they don't need a template for their patterns. They're not directed by something living. They just happen. All of that beautiful, artistic complexity is the result of simple, mindless processes. I find that enthralling. The power of physics, chemistry and geology to combine and form such patterns is amazing.
Wonderful ol' world, innit?
"Liesegang pattern development in carbonating traditional lime mortars." Rodriguez-Navarro et al, Proceedings of the Royal Society, 2002
A version of this post first appeared at En Tequila Es Verdad