Moisture is Our Enemy
Over the years I have discussed cutting meteorites several times. So I am not going to go into that part too much again. But, it has been a long time since I wrote about the handling of meteorites during and after the cutting process.
None of us are going to take any of our meteorite slices and put them under the faucet and run water across them. But in a very real sense that is what happens to them when they are cut. So it seems important to go over procedures to make sure that the exposure they have to water in cutting is handled to prevent later problems.
Chlorine is the major offender in the local water supply for most of us. So I use purified or distilled water when cutting. But, I am looking into obtaining a small filtration system that removes chlorine along with other chemicals as an alternative to buying distilled water. There are several on the market that are good for this purpose and not real expensive.
I always add some alcohol to the cutting water to provide a wetter solution. Alcohol reduces the surface tension in the water. It will cling to the blade and specimen better. It is not really a lubricant additive but does help to keep good contact to cool the cutting. I have stopped using alcohol as a bath after cutting a slice since I am using distilled water. I maintain a very strict procedure to dry the specimens and that is where we will head now.
I do the cutting for myself as well as the Meteorite Exchange in batches. Unless a stone is really intriguing I will not just make one cut. When the day comes to do cutting I will lay out the stones and plan the way each will be cut and run them all through the saw in quick order. As each slice is cut off it is dried with a towel and plunged into a large can of fresh silica gel. Each of the slices of that meteorite are treated the same. After completing the cutting of a stone the slices are collected from the silica gel and put in a smaller airtight container with silica gel and a label of what the meteorite is. The slices when they are removed from the large container are bone dry. There is no moisture in them at all. The important thing here is that the silica gel be fresh or newly recycled by drying it out slowly in an oven. If it is not fresh you will not pull all the moisture out of the slices as they are cut. I put the metal lid on the large container after each slice is plunged into the gel to prevent moisture in the air from also being absorbed. This is a tiresome procedure, but necessary. After all the slicing is done and all the meteorites are in their individual containers with silica gel I begin the lapping and polishing process.
Surface of 2.2 kilogram half mass of 99676 eight years after cutting. It has been sitting on a shelf unprotected. Metal is bright and shiny and surface is dry and free of any alteration.
I work through the different grits of diamond laps again in batches. I do both sides of each slice and plunge them again into the silica gel after they are finished. Some will be polished some will not. Some will end their grinding at 400 grit or 600 grit. Others will go on to 1200 grit and polishing. It just depends on the particular meteorite and how its features are best shown. Polishing often enhances the characteristics but not always. Some meteorites lose so much contrast when polished that almost nothing can be seen. It is better to leave these with a fine ground surface. The ones that are polished will need a more intensive cleaning after to remove the residue of polishing compound. A brush with stiff plastic bristles will work fine for this even an old tooth brush.
The important thing for this discussion is that at every stage the slices are dried. I never allow the slices to lay around wet waiting for the next step. After the slices are finished they are kept in silica gel until they are weighed and packaged for selling. That may be a riker box or one of our other display units. The finished weighed product will then be placed in a container with silica gel and a relative humidity indicator card.
What about the meteorites on display? I can not display most of my meteorites without risking damage, because I live near the ocean. So I do not display sliced stones or irons. A few with cut windows are on display. Most of these were cut dry in the open air with the dust blowing free from me and lapped dry as well. Dry cutting is bad on the blade and the dust not good for us to say the least. That is why it is done outside the garage. But, if I am going to have the stone on display then it has become my practice to never let it have contact with water.
I have many complete stones on display. They are beautiful most were inexpensive. Most on display are NWA meteorites obtained in the last few years. Some have old breaks on them. I may grind a broken surface flat on a lap to enhance the appearance of a stone, sometimes nothing is done to them.
What about my collection? How do I control moisture for it? Well, that is an entirely different set of problems. The collection is in dry boxes containing large supplies of silica gel with indicator cards. I am not in the boxes all the time so recycling the silica gel is only a periodic chore. But, it is something that does have to be done. I use a woven cloth tubing originally created for the metal plating industry to hold the silica gel. I cut a length of the cloth tubing off a roll and tie a simple knot in one end then fill the tubing with fresh silica gel and tie another knot. This bag is very porous to allow free movement of air. But also tight enough woven to contain the large grains of silica gel. There is a problem of silica dust. I place the bags so this dust does not get on the meteorites and make scratches during handling. Holding a pound or so of silica gel, each bag can absorb a great amount of moisture. I keep two or three such bags in each dry box. I have not had many problems with my meteorites. I do have a couple that are just really stubborn about staying corrosion free. I think the problem with them however, goes back to the way they were cut and etched by the original preparer. They are irons with cracks and inclusions. I donít think a good job of neutralizing and removing acid was done. I know that I will have to go back and redo these at some point but as for now I have them in a good state of stasis in the dry boxes.
I have never had a bit of rust on a complete stone in my collection and none of the slices of stones has been a problem either. All that handling through the cutting process is tiresome, but has served me well over the years and made for a worry free product to sell.
I am going to be performing this ritual again very soon. I have begun planning the cuts on several large stones that have to be sliced up. So it is time to recycle a large batch of silica gel. Now it can be done in a microwave but it is risky. If the silica gel gets too hot it will have its micropores damaged. It will not absorb moisture well. The much better solution for recharging silica gel is to use an oven. It takes a lot of time. The gel does not give up its water very quickly. But, over the course of several hours it will become dry and absorbent again. Most manufacturers recommend several hours at a temperature of around 250 degrees f. This has always worked fine for me. I empty out the cloth bags by untying one end. I pour the contents onto a cookie sheet. Then place it in the oven and forget it for four or five hours. It is important of course that once it is out of the oven that it be put immediately into a sealed container. It will be absorbing moisture fast in the humid kitchen. I use the metal cans that the gel comes in originally to hold the recharged gel as well. I recommend putting an indicator card in the container to monitor the gel. As you open the can repeatedly to obtain a supply for packing with meteorites or for placing hidden gel in a display it will be absorbing moisture from the air each time. Gradually it will even in the can become laden with water. A humidity indicator card will show you where you stand with the absorbency of your gel. The indicator cards come in a variety of ranges usually with three spots on them. I obtained a few hundred of them years ago that are odd but nice. They are 5%, 10%, 20% cards. They are much more sensitive then the normal 20%, 40%, 60% cards. Indicator cards can be obtained from desiccant suppliers online. They work well and are much more convenient then expensive humidity monitors.
Silica gel comes plain and as an indicator type which contains cobalt chloride the chemical that changes color with moisture. The chlorine in it has always made me leery of it since as I have described there is a lot of direct contact of the meteorites with the silica gel. Its use as a desiccant only may hold little danger to meteorites but I just have not used it myself. Silica gel is also sold as cat litter now and very available. No need to find a desiccant supplier anymore just a grocery or pet store.
I actually have a couple of meteorites permanently entombed in silica gel. First, is a large Nantan that has a beautiful crystalline structure, (the Widmanstatten pattern in 3D) but it is very susceptible to deterioration. The second is a nice Nantan individual with an etched window. Both of these I fear would not last long in free air. So they are prisoners of their poor reputation, immersed forever in silica gel. They might get out someday if I create a plastic display box that is climate controlled to protect them, but for now they are safe in a metal can.
It would be nice to have some slices on display, but I have a great desire to keep all my meteorites nice and preserved for the future. Display for many is just not going to work since climate control of my office is an impossibility. The fusion crust on the whole stones is a great protector of the insides under normal moisture conditions. And they are not handled without gloves for the most part. Though I have to admit I may let a grandchild hold one once-in-a-while without gloves.
Preserving our collections is something we should all be thinking about as we are only the caretakers of this material for the length of our lives. How we turn them over to the next generation is something to consider. I know that I am grateful to some unknown individuals who took great care of specimens I now have. I would like to think that my collection will be in fine condition for the many more years (hopefully) that I have it, and that it will still have its total value both scientifically and monetarily when it is transferred to new owners. That preservation all begins with controlling moisture and the human touch.
Its been a while since I offered an installment of Scale of the Month so I will make one this month. In my ongoing attempt to buy all the surplus laboratory scales on the market I found one recently that was quite interesting. Over time all the companies that made scales went from balances using weights to scales with internal weights at are applied to the beam by means of levers or cogs. Eventually they have all gone on to make digital scales. This one is one of those transition scales by The Torsion Balance Co. It has the more or less normal torsion scale mechanism, but a set of weights are suspended on hangers able to be raised or lowered by a series of cogs. As with the other companies an engraved glass scale on the end of the beam moves in a light path and its graduations are projected onto the screen at the front. Nothing new there all about the same from manufacturer to manufacturer. What is different is that instead of making the fractional gram reading by deflecting the beam through shifting one of the mirrors. This Tobal has a vernier scale on the front screen that gives a direct reading of tenths of a grams. Since it has a capacity of 1 kilogram with the normal counterweight and a 2 kilo capacity with the larger supplied counterweight more precision was not deemed important. In fact most 1-2 kilogram scales have only 1/10 gram graduations.
The vernier scale is the interesting part of this machine. By employing its use a very complex mechanism used on other scales has been eliminated completely. Vernier scales have a long history. Beginning with devices like sextants and continuing through to calipers and micrometers. Generally any precision device giving an indication on a scale could be fitted with a vernier scale for more accurate reading. The following picture of the indicator will show how it works visually. For those unfamiliar with vernier scales see if you can figure it out by looking at the picture. Enjoy.