Read about how Mike set about his task to discover the properties of gold.
Day 6 - Properties of Gold
So, here we go again, our first day of filming for the third series of Rough Science. It’s great to be back with the old team. Although we arrived in New Zealand last Sunday, we’re all still feeling the after-effects of the 20 000 km trip out here. My first impressions of this place are that (i) the people are all so friendly, (ii) the country is so beautiful, and (iii) I’m so damned lucky to be here.
It’s going to be a fairly easy day today. Our first job is to be flown in two helicopters over the Franz Josef Glacier - now, is that a great start to the day or what? The glacier and the surrounding alpine scenery are indescribably beautiful. To see it so close up from the air leaves me speechless.
The helicopters take us to the sawmill, where we are given our challenges for Programme 1. Ellen and I are asked to locate a site where we can pan for gold - yes, gold. There are tonnes of it up there in the mountains, most of it is embedded as tiny flakes in quartz veins in the rock. Weathering and erosion release the gold from the quartz and surrounding rock, and it’s washed downstream by glacial melt-waters, streams and rivers. On its journey to the sea (amazingly, the coast is only 5 km from the glacier here) the flakes are broken up into finer and finer particles. Gold is a very unreactive metal - it doesn’t corrode or rust like many other metals. Its extraction is therefore just a matter of physically separating it from the rocks, gravel and sediment on the riverbed; no chemistry’s necessary. However, any gold to be found here will contain copper, silver and perhaps platinum as impurities.
The site Ellen and I are looking for from the helicopter is something like a bend on a largish, fast-flowing river. Because the water flows less quickly on the inside of a bend, it’s here that the heavy gold flakes and grains will be deposited in a line (anything from a few centimetres to a metre or so wide) on the riverbed. Another place to look is under large boulders in the river. As the water carries the gold and sediment over a boulder or rock, the gold grains and flakes fall to the riverbed on the downstream side.The gold eventually works its way down the clay bedrock, where it comes to rest. A drop in the water level allows access to these so-called alluvial gold deposits.
Having located a suitable site on the bank of the Whataroa River to start our search, Ellen and I agree to take samples from various positions in an attempt to locate the line where gold might have been deposited. We use an old prospecting technique called panning. This involves scooping up a handful or two of the sediment from the site in question into a wok-sized pan, and swirling it about in water, all the while agitating so as to work the heavy gold particles to the bottom of the pan. After a few vigorous swirls, the bulk of the larger pebbles and gravel can be swept off the top. The swirling process is continued, each time removing lighter and lighter material from the top of the pan. You eventually end up with the darker, heavier silt, most of which is the black mineral magnetite (a magnetic iron ore). It’s at this stage that you have to be more careful and swirl the contents of the pan more gently. If you get the right action, and if there was gold in your original sample, a fleck or two of gold grains (or if you’re really lucky, flakes) will separate out from the dark material. When it happens, you get the most amazing buzz - gold fever!
Day 7 - Properties of Gold
We’re all up early for a 45-minute drive down to the river - no helicopters today, unfortunately. Unlike yesterday, it’s tipping it down, and what’s worse, it’s the kind of rain you get in the Lake District in the UK - unremitting drizzle that soaks you through to the bone. We are, after all, in a tropical rain forest, so I guess you’d expect a bit of rain. In fact, Kate H told us yesterday that this part of New Zealand gets 5m of rain a year. Looks like today’s the day all 5m are due to fall. To make matters worse, the air is thick with sand flies, whose bites are vicious, and even draw blood. The balaclavas and insect repellant we’ve been given provide little defence against them. We’re just going to have to grin and bear it. Nothing else for it.
Mikey L’s joined us this morning, which is a relief, because there’s lots of shovelling to be done if we’re to recover any gold. The aim is to revisit those sites on the riverbank that showed the highest number of flakes during the panning process yesterday. This is where we’re most likely to find the yellow stuff. Ellen and I have decided to try a technique that’s commonly used by gold prospectors - sluicing. It involves shoveling sediment from the riverbank onto the top of a sluice box, and passing water down the inclined box so as to fluidize the sample. It’s a gravity separation technique much like the panning process, in that the lighter sedimentary material is swept down the sluice by the water, while the gold, being much heavier, falls to the bed of the box. Ellen’s found a large fern on the riverbank, called a punga tree, whose ‘bark’ will make an excellent trap for the gold. It has the same kind of fibrous consistency as a loofah. By laying pieces of punga bark across the sluice bed, we’ll be able to filter off the rough-edged gold grains.
Mike sets about building us a sluice box, while Ellen and I prepare the punga baffles. An hour later, we carry our sluice box down to the river, and we’re ready to start shovelling. This is a high-throughput process - the higher the better. By the end of the day, it feels like we’ve turned the entire riverbank over. My back’s aching, and I’m soaked through. We’re all still smiling though, probably because we’re surrounded by some beautiful, forested countryside. The river water’s crystal clear, and, thankfully, not too cold.
Day 8 - Properties of Gold
The whole team’s come down to the river with us this morning, which means we can double the scale of our operation. Mike knocks up a second sluice box, and we all lay in to a morning of backbreaking work. Gold fever takes over, which is just as well because it’s raining even more heavily today, and our ‘waterproof’ clothes are still damp from yesterday. We happily call it a day at lunchtime, carefully dismantle the sluice boxes, and take the recovered punga bark back to the sawmill. We aim to burn the bark, something that will have absolutely no effect on any gold that’s been trapped in its fibres. What we’ll be left with is a pile of ash containing any gold we’ve managed to collect. It’ll then be a simple process to separate the heavy gold grains from the ash, using the same panning technique we used on day one. That’s the theory, but it turns out that the punga bark is so wet that it’s extremely difficult to burn.
It’s late afternoon on the final day of the challenge, and we’re just not going to have time to extract our gold from the bark. We have to settle for using Kathy’s balance to weigh the small amount of gold that we managed to retrieve from the panning on day one, which we’ve combined with the gold recovered when we carefully washed down our sluice boxes this morning. Even so, Kathy’s balance indicates that we’ve managed to extract 0.55g of gold, albeit impure, and this is confirmed by re-weighing it on the ‘more-accurate’ top-pan balance that Kate H has brought along. Not a lot of gold to show for all that hard work, but very satisfying, nevertheless. And we still have what we hope is the bulk of our gold still temporarily trapped in the punga bark. There’s no way of knowing what our total yield will eventually be. If we can get more than half a gram from just a few pans and the box washings, I’m confident that we’ll end up with a few grams of the stuff; certainly enough to work with in future programmes.