How to make cylinders out of salt

One of the necessary steps in preparation for running my experiments is creating cylinders out of salt.

Before I show you how I do that, I'll give you a brief review of our experimental apparatus: The experimental powder is sealed into small (2 mm diameter) gold capsules, which are placed into holes drilled into plugs of MgO (which looks just like a piece of white chalk for a chalk-board, but it contains magnesium instead of calcium). The MgO plug and its contents are placed into graphite cylinders (which we purchase), and the entire package is then put into cylinders of salt. Finally the salt and all of its contents are loaded into a large metal container and placed into the end-loaded piston-cylinder machine, which is what creates the high pressures necessary for our experiments.

Making the salt cylinders is actually kind of fun—it combines classic “science” moments, like using a high-precision scale to measure the ingredients, with serious power tools.

We start with two different types of salt. The “large” grain salt is just the stuff one can purchase in a store to re-fill one’s salt shaker. If you look closely at it you will see that it is comprised of many small cubes of salt. We use a mix of about 2/3 salt powder (obtained by grinding table salt to a fine powder in a coffee grinder (this is very bad for the grinder, and we need to replace them more often than my boss would like to do) with 1/3 of the table salt.

The reason for the blend is so that the different sizes of grains will permit a closer packing than would be possible if everything were exactly the same size. (Try comparing how tightly one can pack marbles or beans into a glass jar if one uses only one size, or two noticeably different sizes). After I carefully measure the requisite amount of salt (just over 5 grams total for these cylinders) I assemble the mold.

The mold is made up of an outer steel cylinder the central hole of which is just over 25 mm in diameter, an internal steel shaft which is 8 mm in diameter, and two end pieces which have holes just large enough into which to insert the shaft, and which are just the correct size to fit within the outer cylinder. One of the two end pieces is in two parts so that the larger end won’t fit into the outer cylinder. The other end piece is a single size so that it can pass entirely through the outer cylinder.

After carefully spraying all parts with a Teflon lubricant, the outer cylinder is placed upon a metal washer upon a sturdy metal platform. Then the small end piece is placed inside, and the steel shaft is placed within that. I then fill it with the salt, using a small rod to tap it down around the shaft and be certain that I’ve eliminated any large air pockets. Once all of the salt is between the outer cylinder and inner shaft, and has been brushed off the end of the shaft the two-part end piece is added to the top.

This is where the power tools come in. Human strength might be enough to push that end piece part way into the hole between outer cylinder and inner shaft, but it would never be enough to cause the salt to recrystallize and adhere to itself and become a single, cohesive mass. Therefore I place the metal platform upon which the filled mold is standing onto the base of the hydraulic press and very carefully align the mold with the pressure rod.

Once everything is positioned exactly correctly I close the door and engage the motor, which forces the pressure rod down, driving the end piece into the outer cylinder, pushing the salt out of its way before it.

When the end piece is fully inserted and the rim is in contact with the cylinder (and before the pressure starts to increase because it can't move any further), I stop the load, retract the pressure rod, invert the entire mold stack, and set it back down without that above mentioned washer (which was used for the sole purpose of causing the smaller end to stick up out of the outer cylinder by a few millimeters after the first end had been inserted).

I then carefully align that smaller end piece with the pressure rod, close the door, and engage the motor to drive the rod down onto the end piece, which pushes the salt from the other direction into the center of the mold. I hold the motor on until the end piece is fully inserted and the pressure dial just starts to rise due to the resistance it is now encountering. Stopping the motor before there is too much pressure is critical—it is possible to do major damage to the mold or the machine (or both) by ignoring the gauge and continuing to apply force after the goal has been achieved.

The salt cylinder now exists, but it is caught fast within the outer cylinder, as are all of the other parts of the mold.

Therefore it is necessary to remove the large metal platform upon which the work has been supported, and set the cylinder over the smaller platform with a hole in the center. Then the pressure rod can once again be lowered, where it will slowly push the salt cylinder, internal metal shaft, and small end-piece down through the outer cylinder, through the hole in the underlying platform into (padded) chamber beneath.

The cylinder is now nearly ready to use. As it comes out of the mold it is just a tiny bit too large to fit into the metal container for the experiments. Therefore we mount the shaft (which is still inside the salt cylinder) into a lathe (after first wrapping both ends with some tape—to protect the metal from the clamp on one end, and to prevent the salt from spinning off the other end).

While the lathe is turning I carefully use a small bit of sand paper to polish the outermost layer of salt off of the cylinder, stopping my work often to check to see if it fits yet.

If I were to sand off too much, I’d need to start the whole process over.

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.