Learning to create capsules for experiments

Having recently started a position as a post-doc with an experimental petrology team it is now time for me to make the transition from reading a huge stack of background material to actually doing experiments. This requires learning a whole new skill set, like welding. Here follows the notes I’ve made on the first few steps of creating sealed capsules full of material to be taken to elevated temperatures and pressures during my forthcoming experiments. This is mostly for my own information, but someone out there might find it interesting or useful (or have helpful comments they can add), so I’ll share it here.

*Step one: Prepare the holder in which the capsule will be placed while filling it

During the filling process we use small metal disks into which holes of varying diameter have been drilled as a holder for the capsules (different sized holes are needed because different experiments use different sized capsules). First find a disk which has a hole with the correct diameter (or make a new hole in a disk if necessary). It needs to be just big enough to insert the tube into, without being loose. Then use fine sandpaper to carefully polish the metal around that hole so that when you get to step 6 you will have an easier time of filling the capsule.

*Step two: cut the tube for the capsule

For my experiments I will be using capsules made of gold tubing that is 2 mm outside diameter (1.4 mm inside diameter).

Obtain the correct diameter and metal tube (our lab uses both gold and platinum/gold alloys in a variety of sizes, I’m to use gold for mine) and then cut off a ~7 mm length from one end. To cut the tubing place it on a metal plate, then place an x-acto blade upon the tube and use the blade to exert a gentle pressure to roll the tube back and forth until the blade cuts through without squishing the tube. The back-and-forth motion is essential. This is not “sawing”, which uses a serrated blade to tear chunks out of a material which is stationary, but rather the tube itself rolls during the process as the blade slowly cuts into it.

*Step tree: Pinch closed one end of the tube

To seal the first end of the tube use pliers which have slightly rounded side edges. It is important to use the correct tool, as the sharp sided pliers can pierce a hole in the gold between the unpinched and pinched portions of the tube. (However, one of my colleagues uses the straight-sided ones, but he is very, very careful.) We use a three-part closure, which looks much like a symmetrical peace symbol. In order to create this do it in stages, don't try to squeeze it to final tightness on the first go, that won't work. Instead do a little at a time, slightly pinching on one side, turn the tube 1/3 of a rotation and pinch again, repeating around the tube, tightening the pinches a bit more each pass. Ideally, one wants it to be closed all the way to the center without leaving a hole at the triple junction. Unfortunately, this is difficult to achieve when working on such a small tube with pliers which are so much larger than the tube. Once the end is pinched shut trim the three edges using a cutting tool with beveled edges. The reason for wanting beveled edges to the cutter is so that the place where the two edges of gold meets is as narrow as possible, which will make welding easier. Hold the cutter at a slight angle so that once trimmed the triple point at the junction is slightly higher than the far ends of each of the three seams. (This photo, above left, taken through the microscope, shows a crimped tube held in place in the clamp and ready to weld.)

*Step four: Weld the pinched end shut

The voltage necessary for welding will change based on a variety of factors, including the diameter and length of the capsule, the thinness of the seam, the sharpness and length of the graphite in the welding tool, and what, if anything, you use to cool the capsule as you work. Unfortunately, our

welder isn't very precise and it can be difficult to adjust it to the perfect voltage for any given job. For this size we tried a variety of settings between 25 and 30 V, the 25 V was clearly too low--the welder left it looking "dirty" and coated with black, which is graphite from the welder being left on the gold. At 30 V it was too high; there is too much melting. In between that range the exact value was hard to find, and as variables change, so does the perfect voltage for the task. One variable which can make a huge difference is the sharpness of the graphite point. We have two different sharpeners, one of which makes a sharper point than the other. Using a “point” created by the duller of the two sharpeners at a voltage which isn’t high enough for that point and then switching to a point created by the sharper of the two sharpeners without turning down the voltage will result in the entire end of the tube melting.

The bit of advice I obtained the next day seems to have made a difference—don’t try to touch the gold with the graphite point, but rather hold it just barely close enough to cause an arc between them, and then try to draw that arc along the length of the seam. This isn’t easy, but I did wind up with useable results. Alas, the photo to the upper left doesn't show the welding very clearly--gold is just too darn shiny to photograph well through a microscope with a cheap camera when resting in a brass holder (this is after adjusting the brightness/contrast/intensity to make it visible at all).

I was also told that when welding I should try to start at the outer edge and draw the graphite point towards the middle, which brings excess gold from the edge towards the center to fill the small hole at the triple-junction. The goal is also to wind up with a flat bottom after welding.

*Step five: Prepare the welded tube for filling

After welding the tube it is necessary to re-shape the tube so that the capsule will have properly rounded/curving edges. We have a form (photo to the left) into which the tube is placed carefully so that the widest parts will be inside the form and not pinched between the two halves of the form (one chooses the correct diameter chamber within the form for the tube in question, of course). Once it is positions correctly the form is closed, and the tube is pressed back into a cylindrical shape. Once it has been re-shaped in the form it is put it into the holder (prepared in step one, and resting on a metal plate) and insert into the gold tube a small rod which has a diameter which just fits into the tube (in this case the rod needs to fit into a space 1.4 mm wide). Gently tap the rod with a mallet so that the bottom of the capsule flattens against the underlying metal plate and spreads out to match the curve of the sides of the hole in which the tube rests (take care as to not strike it so hard as to tear a hole in the gold tube!).

After much effort I now have three small capsules with one end of each sealed and flattened, and the other end still open and ready to fill. Stay tuned for steps 6 and 7 once I get them working. Having finally managed to get the tubes ready to fill, I chose to rest on my laurels and call it good for the day.

Pre-Conference Field Trip, Stop Two: Siccar Point

It has been nearly two weeks since I attended the Pre-Conference Field Trip, yet the memory of the day remains strong. I have already written about Stop One; today’s post continues our adventures as we follow in the footsteps of James Hutton, visiting the unmistakable unconformity at Siccar Point.

This outcrop is said to be one of the most significant in the history of the study of geology. Before Hutton published his theory on geologic time many people accepted Bishop Ussher’s calculations that the earth was only 6,000 years old. However, Hutton’s observations of geologic phenomena led him to realize that it must be far, far older than that to account for the sedimentary record. He observed erosion taking place in the world around him; saw how much sand a river can carry to an ocean over time, and reasoned that this process must have been going on for as long as there have been rivers. The rate at which the sand is deposited onto beaches or into lakes, or along a riverbed is measurable, and so estimates of the time needed to deposit a given thickness of sand may be calculated. He also noted that when sand is deposited by water, it always happens in horizontal layers. Comparing such layers of fresh sand, mud, and/or gravel with layers of sandstone, mudstone, and conglomerate leads one to the realization that the sedimentary rocks must have, at one time, been made up of loose sediment, before they became compacted and/or cemented into solid stone, and any sedimentary rocks which are folded or tilted must have been tilted or folded at some point after deposition and after becoming rock (or the sand would have slid back down into flat layers again).

The rocks in southern Scotland include two very different packages. The older of these packages is made up of Silurian sediments—poorly sorted sandstones (greywackes) which have been folded intensely enough that the bedding now stands on end in many locations rather than horizontally. The younger package is the “Old Red Sandstone”, which was deposited during the Devonian. Hutton knew from his explorations in the region around Edinburgh that the red sandstones are reasonably flat-lying and outcrop to the north of the steeply tilted Silurian greywackes. He knew that there must be some place where the two rock types are in contact with one another, so he and some friends set out in a boat along the coast in search of it.

One can imagine the delight felt by his party when they discovered that contact, at Siccar Point. While it had been possible that there was a gentle transition from the steeply dipping sedimentary rock to the south into the flatter lying sandstones to the north, what they discovered was no gentle transition.

Instead they discovered evidence that the older rocks must have been deposited as typical flat-lying sands and muds of great thickness, transformed into rocks and folded so tightly that the layers now stand on end, the edges of the folds and some unknown amount of rock eroded away, and then, some time thereafter, the sands which were to become the “Old Red Sandstone” deposited atop them, before themselves becoming rock and then being subjected to erosion. Hutton argued that if sedimentary rates in the past operated on the same sort of time scales as they do today there is no way that all of that could possibly happen in so short a time as only 6,000 years. One of his companion on that trip, John Playfair, is recorded as having said that “The mind seemed to grow giddy by looking so far into the abyss of time” when they contemplated just how much time the entire process would have taken.

Indeed, what they couldn’t know as they stood upon this rocky shore contemplating "deep time" was just how much time is represented by just the period of erosion between the two rock units, which has subsequently been calculated at fully 55 million years, during which an ocean basin closed and a mountain range grew.

Unlike Hutton's first boat-trip to this location, our field trip approached the point by bus, traveling south from Edinburgh, and finally stopping in a field near the coast some 55 km to the east of Edinburgh. Travellers are greeted at the stop with a first information sign at the trail head,

and a second when one reaches the point at which one descends to the seaside.

The way was steep, but, fortunately, it wasn’t a rainy day, so the grass was dry, and we made our decent safely.

We then spent a pleasant time examining the rocks, and taking photographs before climbing back up the hill to return to civilization, content in our own observations that yes, indeed, this outcrop does record simply amazing amounts of time.

A tale of two Conferences

Like many scientists, I have a variety of interests, in some pretty diverse fields. The past two weeks were spent indulging two of them by attending two very different conferences.

The first conference, on Micro-Analysis, Processes, Time, was sponsored by the Mineralogical Society in the UK. Like many geological conferences, it began with a pre-conference field trip, the first stop of which I’ve already posted (and will be getting back to post about the other stops soon, now that I’m home). The conference itself had around 200 people attending (small compared to some!). The schedule was fairly typical for geology conferences, with talks running during the days (generally business hours), and people scattering to the four winds in the evenings. There were enough people presenting that Monday and Wednesday had four tracks of talks happening at once, meaning that one had to pick and choose amongst them, but, generally, things were arranged such that it is likely that if there is one talk in a session one is interested in, the others may well also appeal, meaning that I was able to simply find the appropriate room in the morning and stay put all day, rather than trying to bounce from one session to another, and risk missing the start of a talk in another room. Tuesday morning there was a single track of talks, followed by the poster session. Since my contribution for the conference was a poster, I took the time during lunch to read the other posters, so that I’d be available during the session to speak with people reading mine. I met a variety of interesting people during that afternoon session, but only had a short chat with each of them. After the conference there was a post-conference field trip (post with photos to follow). I did meet a few of my fellow field trip travelers, but the bus ride was mostly quiet or people speaking with their seat-mates, and, as it turned out, I didn’t have a seat mate.

The second conference, the Textile Forum, was quite a contrast in organizational style. This conference was set up as a setting to an experiment. The experiment was designed to quantify the changes that result in the quantity and quality of yarn spun with drop spindles with different masses and moments of inertia. Therefore the experimental part of the conference involved two one-hour sessions of spinning a day for five days in a row—each session using a different spindle whorl. As a result this conference worked on a very different schedule than the geology one. There were roughly 20 of us in attendance, from a variety of different countries.

Most of us stayed on site in the Iron-Age style “hut”, which is equipped with straw mattresses and sheepskins to supplement such bedding as we brought with us. Therefore our day would start with a communal breakfast, followed by a session wherein the spinners did their two one-hour sessions (with a break in between) and the rest of us worked on other hand-crafts and everyone chatted about any number of topics of interest before lunch, then after lunch we’d scatter to the four winds (many of them to see museums), and reconvene in the evenings for the talks, followed by social time spent around a campfire before sleeping.

The combination of the very different time-schedule for the conference, with the much smaller number of attendees resulted in the opportunity to get to know my fellow Medieval Textile enthusiasts much better than I did any of the geologists at the first conference. I will be happy to recognize the geologists at all next time I see them, but I now call many of the textile people friends. In short, while I found both conferences very enjoyable, with many interesting things learned, I think that I liked the format of the textile conference much better. Alas, off of the top of my head I can’t really think of a way to do a parallel sort of thing in the geosciences. What task do geologists do that would permit them to gather in a small group around a table, doing said task while conversing with one another? And what would it take to get them to do it? The spinners did their spinning (with some very oddly shaped/sized/weighted spindles “in the name of science” to see if there is a difference in how spindles behave which isn’t directly related to the spinner. But when geologists do experiments it usually involves setting something up and then going away for a period of time. This practice wouldn’t be conducive to a similar environment as was achieved in the Textile forum.