Monday, 22 February 2010

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.

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