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specific gravity

Posted by Gord Howe  
Gord Howe January 27, 2011 02:29PM
Quite some time ago I stumbled onto an article on specific gravity on the internet that I had to leave before I got to read it all the way through, and, of course, I didn't bookmark it. The article covered the usual methods and contained info on building your own gear. I believe the article was written by Alysson Rowans. I have not been able to relocate the article. If anyone can point me in the right direction I would certainly appreciate it.
Reiner Mielke January 27, 2011 02:46PM
John Sinkansas's "Mineralogy for Amateurs" has a section on building your own SG balance which I built when I was a kid, worked great. You might want to try that.
Stephen Johnson January 27, 2011 05:38PM
John Bett's gave me a tip about two years ago that I've found incredibly useful. It's so simple and so easy to do and actually fairly accurate - at least for my purposes. You don't need all the fancy equipment that's sold unless you're doing detailed lab work. It's so easy in fact that you can do it while they're in the field. All you need is an electronic scale and a container to hold water. I usually set my scale to read "carats" rather than "grams" or "ounces". Since the S.G. is a ratio to generally two significant digits, setting it to "grams" works out fine. Also, since it's only two significant digits - just assume the the S.G for the water is "1" without factoring in any temperature difference from 20C.

First measure the weight (mass) of the sample and write the number down.

Second, place the container for water on the scale and zero it out. At this point you're going to dip your sample - I've used dental floss in the past to hold the specimen - but have since worked up a little thin aluminum plate with string (dental floss again). I usually dip the plate in the container as well and re-zero. I then place the the sample on plate and dip it in the container and take my second reading and write the number down.

At this point it is simple math. Take your first value and divide it by the second. This will give you a S.G. value that is close enough for most of what you need to for identification...along with other characteristics like hardness, cleavage, crystal system, etc. This is a simple and quick way to get at a S.G. for most identifications.

If you need to get more than two significant digits, then go ahead and knock yourself out with a more elaborate and expensive system.
Gord Howe January 27, 2011 06:35PM
Thanks to all of you.
David, that is the article I've been hunting for. Reiner, I will try and find this book, not sure if its still in print. Good info Steve, I know I'll be refering back to it once I get started.
Thanks again
Kelly Nash January 27, 2011 06:52PM
Yes, I use John Bett's method as well. It's very easy and pretty accurate. I use a digital postal scale that's available for $10 - $15.

The method was posted on his (John Betts Fine Minerals) website homepage on 6/9/09, and is still there in his archive. (I'm taking the liberty of pasting it below):

"I have developed a simple technique for determining specific gravity using a digital kitchen scale:

1. Turn on power and the scale should "zero" itself.
2. Weigh the specimen and record the weight.
3. Find a container large enough for the specimen and fill with water with room enough to submerge the specimen.
4. Place the water-filled container on the scale and "zero" it out.
5. Suspend the specimen from a wire or paperclip in the water, but not touching the bottom or sides.
6. Record the weight reading.
7. Divide the weight in #6 into the weight in #2 and you get specific gravity.

Do not ask me how or why it works. The explanation will only confuse you. All you need to know is that it works, it is fast, it is easy and does not require any lab. equipment. This technique is accurate to the first decimal place. And that will narrow down the possible mineral species of an unknown specimen making it fairly easy to get an accurate identification in your own home without spending money for testing."
Peter Haas January 27, 2011 08:03PM
Accurate or precise ?
David Von Bargen January 27, 2011 08:04PM
John Sinkansas's "Mineralogy for Amateurs" is not in print, but I keep hearing that there will be a new edition of the book, For people who are serious about mineral id, Don Peck's book is excellent (published by Mineralogical Record)
Alysson Rowan January 27, 2011 08:17PM
You should find not only the how to's of using a range of methods for determining specific gravity (relative density) in my article, but how to construct the instrumentation.

The Jolly Balance, for example, if the spring is carefully matched to the sample size, is capable of a precision of the order of better than 0.1% - and that is just using scrap materials and common workshop tools. At that level of precision, the difference between tap water and distilled water will be measurable.

Please remember that the article is a work in progress, and is still not actually complete (yet).

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Kelly Nash January 27, 2011 09:10PM
I believe accurate is the right term for the shortcut (John Betts) method, because it is fairly reproducible, and my measurements of known minerals compare well with published data. It is not precise because it only weighs to 0.1 g and so most determinations are only good to two significant digits. I don't doubt at all that a Jolly Balance is more precise, but really, if your tap water introduces a 1% error it probably has a darn high dissolved solids content.
Peter Haas January 27, 2011 09:27PM
Speaking of "reproducible", you throw in another term with yet another meaning. In fact, there is no way for you to determine the reproducibility of the technique, but you might well have determined its repeatability.
Alysson Rowan January 27, 2011 09:43PM
The difference between tap water and distilled water will produce a difference in SG of up to 0.1%, depending on the levels of dissolved carbonate. With an accuracy to about 0.1%, a 0.1% difference in working fluid SG is quite a significant detractor from the precision of the technique.

Of course, one must always state the worst-case figures for the uncertainty of a reading. By making your corrections and by using a standard sample (usually a glass bead of known SG) you can reduce the errors introduced by using impure water by a simple correction.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Kelly Nash January 28, 2011 12:02AM
Sorry, I stand corrected on my misuse of the term "reproducible", I did mean repeatable.
Mineralogical Research Company January 28, 2011 12:30AM
To be even more "specific", the temperature of the water is a factor. Over a moderate range of room temperatures, say +20C to +30C, the density of water will vary a couple tenths of a percent.

Gord Howe January 28, 2011 12:33AM
Alysson, it was the Jolly that caught my eye the first time around and I remember thinking how easy it would be to build. I'm looking forward to the rest of the article when your done! Kelly, I have used digital scales before along with a 50 ml grad with OK results, but I gotta wonder about how close I really am when I do that. I will try your method next time I get the chance. The specimens I have to use with the grad have to be the right size-too small and I can't see the difference in the grad reading and too big and they won't fit. I'm sure there are sample purity issues with larger samples, as well. With the Jolly all that goes away. Should be able to use whatever size I have providing the spring is the right sizing. Can't wait to start.
Thanks Again
Mineralogical Research Company January 28, 2011 03:02AM
Hypothetically, I have completed doing the same experiment as Kelly. We have compared data and it agrees to, say, a standard deviation. Is it OK for Kelly to now use the term reproducibility?

Alysson Rowan January 28, 2011 10:23PM
As Gene correctly states, the temperature of the working fluid is also important.

Other factors that can affect results to a greater or lesser degree include the amount of air entrained in the fluid ans how well you ensure that there is no air trapped on the surface of the sample. Size, shape and surface texture of sample fragments can have an effect - and if air is trapped on the suface of the fragments (poor wetting) it is possible for the experimental error (correctly known as experimental uncertainty) to exceed the results obtained - rendering your effort useless.

I have seen the SG of lead (dust shot) appear to be only marginally above that of water.

Good technique and consistant methodology counts for an awful lot when performing any scientific measurements.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Dana Morong January 29, 2011 06:01PM
I have done SG tests using the basic method outlined in John Sinkankas' book "Mineralogy for Amateurs" and have used tap water which contains a lot of calcium, and it was at about the temperature of the ground around the bottom of the well. The way I tested its accuracy, was to simply test its calibration: I first tried a couple of samples (of fairly pure substance, and of roughly the same size; a good clear quartz crystal of about the same size is handy - also use the same type of thin thread that is planned for the unknown) that I knew the SG of, then compared them to the results I got. I was therefore able to figure out the amount of error, and whether that was acceptable for my purposes or not.
Donald Peck January 31, 2011 03:51PM
The method for determining density that we all learned in school (determine mass in air, suspend object in water in a graduated cylinder) is not very accurate. The major problem, as was cited by Gord, is the uncertainty of the change in level of the water in the graduated cylinder. John Bett's solution is much better. And I like the idea of using dental floss to suspend the object (I use either silk or polyester thread) . Steve's ise pf a small aluminum plate gets rid of the hassle of tying knots - - I like it.

Alysonn, I haven't seen your paper yet, I need to do that. Jolly balances are great, but the springs are so dog-gonned delicate; and I am fumble fingered.

Peter, I learned that "accurate" is the real value (not entirely measurable as errors get included) and precise is how close do repeated measurements group around a point. A measurement can be wildy inaccurate but be very precise. Of course accurate and precise is to be desired. Am I mistaken?
Alysson Rowan February 01, 2011 12:37PM
On the subject of springs for Jolly balances - for small specimens, you could always use a nylon spring:

Coil some 5 or 10 pound monofilament fishing line around a small resin or wooden former, allowing a small gap between turns. Holding the coil taut (I use a wooden former with holes drilled to maintain the tension, heat the monofilament carefully in the live steam close to the spout of a boiling kettle (don't use a whistling kettle unless you really enjoy the noise!).

Once the monofilament cools and has been run under cold water for a few seconds, you can remove the former and you should find yourself with a robust spring.

Using different lengths and weights of monofilament will give you a good range of springs. These springs will return to size even after being almost stretched straight.

21/2 pound fishing line is probably the lightest monofilament you will find, heavy duty strimmer (weed whacker) line the heaviest. Outside of this you will need to use your ingenuity!

For the sand-grain sized specimens, a curled horse-hair will give good results (but don't let it get wet!)
For multi-tonne specimens, steel hawser would work. (I have seen one of these used with 4mm steel cable for obtaining the net density of rocks and soils for soil-mechanics studies - a very serious piece of laboratory equipment!)

... ... ... and yes, both accuracy and reproducability are extremely desirable. An accuracy to +/- 0.005 g/g(w) relative to water is an ideal target as most mineral S.Gs are quoted at X.XX, i.e. 0.01 g/g(w).

Precision (as in precision engineered), by the way, relates specifically to the systematic reproducability of results when consistent experimental technique is used. (i.e. The equipment produces consistent results which are only affected by differences in the way the equipment is handled?) Reproducability relates to the consistency of your results when actually repeating the measurements.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
TREILLARD Michel February 03, 2011 05:26PM
Hello all,

For Mindat'rs yourselfers, I propose a simple installation of hydrostatic balance. Good luck.
First the ingredients: - a support arm with weighted (eg, old desk lamp)
- A precision balance (around 30 euros)
- A container door set at the end of the arm
- Support weighing up a ring of PVC, with one side a pedestal to be placed on the balance pan, and, unlike a stem for receiving the sample. (I used a welding rod is brass and an old electric socket, which are welded to tin) because the device must not be magnetic ... logical.
We put the bracket on the weighing scale platform, we tare to zero, then we put the sample in the old electric socket. We note the outcome of the measure.
Identical operation, sleeve immersed in the container - do not touch the sides or bottom-. We rebuild a tare to zero. Sample is deposited gently into the socket carrier is allowed to stand so that the system is stabilized and there has been the result of the measure. We have a friendly thought for good old Archimedes helps us well for a successful conclusion to these measures.
We have the dry weight of the sample and its mass in water, then applying the following formula:
Density = Weight in air
Weight in air - weight in water
we get a number that is enough to compare to those of many tables that are found in the literature or the internet, and we have an idea of the nature of the sample. The ideal is a small specimen consists only of the unknown material. Of course this is only an index, further testing will be needed, hardness, color and mineral line, form, place of origin, etc. ...

Bests regard's. Michel
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Donald Peck February 04, 2011 03:16PM
A;ysson, your fishing-line springs sound great! I have to try that. How would a long threaded bolt work for a winding mandrell? (I think it would provide even spacing for the turns in the spring}
Alysson Rowan February 04, 2011 10:31PM
A bolt works well, but it takes a long time to get it up to temperature so that the spring doesn't simply unravel when released. The other thing is that you may find your fingers rather close to a source of live steam! My one attempt with this method was a heavy-weight line that was heated for an hour in a thermostatically controlled oven (120 Celsius is about right)

The other option is to wrap a bare copper wire around a wooden mandrel and then wrap the fishing line over that - it would heat much quicker than a bolt.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Gord Howe February 05, 2011 12:14AM
I was going to wrap the fishing line around a length of about 7/8"(~22mm) ready rod, secure both ends with hose clamps and bake it in an oven for awhile at about 150C or maybe use an electric heat gun to hot form it. I have also considered using a high e guitar string and wrapping it under tension around a mandrel say 3/8 to 1/2" (~10-12mm)in diameter. Its diamter will expand quite a bit when the tension is released but it should hold it shape. I am unclear about the length, assuming the longer the better, and the larger the diameter the better. Also, I'm not sure how to terminate the ends so the load stays as close to the centreline of the spring as possible. Hopefully, I'm way over thinking this.
Alysson Rowan February 05, 2011 04:57PM
150C will do the job, but keep an eye on the filament. A hot-air gun will fuse the filament rapidly and unevenly unless care is exercised. Monofilament, though, is cheap enough for experimentation.

Centering the load is relatively unimportant for a longer spring.

The actual length of the monofilament of the spring is important in that the longer the filament, the larger the displacement under load - however, you must use a rigid enough spring to ensure that it doesn't 'bottom-out' under tension.

You need to experiment, but ideally, you will have a good few milimetres change in extension between dry-weight and wet-weight of your specimen.

The physics of spring extension states that while the change in displacement under load doesn't significantly alter the geometry of the spring, then the displacement is proportional to the load.

What this means in practical terms, is that the spring should open only slightly (less than, say, 45 degrees) between neutral load (just the balance pan) and under maximum load (balance pan and specimen). Under no circumstances should readings be taken when the spring is over-extended, as the larger the extension, the lower the accuracy of measurement.

This limitation is reduced in impact in a longer spring by making the changes in extension larger - but at the expense of using a more rigid spring for larger specimens - a 5lbs monofilament spring will only be useful for a few grammes of specimen at most.

As an adjunct, it is possible to make a simple torsion balance using monofilament that will operate in exactly the same way as a Jolly balance. Whilst it is has a less linear callibration, it is a more robust piece of equipment that requires less head room.

Tonight's homework is to work out why the callibration is not linear.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Rudy Bolona February 06, 2011 12:36AM
I built the one in Sinkankas's book. It has been an invaluable tool. It's extremely accurate, relatively easy to build and inexpensive. I used a pica points rule instead of the graph paper suggested by Sinkankas, this makes it incredibly precise! Paper clips make good carriers and weights which can be selected and cut to different sizes according to specimen weight.
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Gord Howe February 06, 2011 07:44PM
Nice job Rudy! Alysson's article also has plans to build a walking beam balance not unlike yours. I'm the kind that likes to build things perhaps even more than owning them. My toolbox and workshop drawers will attest to that. I decided to "Go Jolly" because I want to learn a bit more about springs and load deflection etc and I'm already wondering how I can use this info for another project. Once I nail down the basics, I will go my own way. Its all about keeping the mind ticking over, and, more importantly, having fun doing it! Having a useful instrument at the end of it all is a great bonus.
Alysson Rowan February 06, 2011 07:55PM
Rudy Bolona Wrote:
I built the one in Sinkankas's book. It has been an invaluable tool. It's extremely accurate, relatively easy to build and inexpensive. I used a pica points rule instead of the graph paper suggested by Sinkankas, this makes it incredibly precise! Paper clips make good carriers and weights which can be selected and cut to different sizes according to specimen weight.

That is a really good looking piece of equipment, Rudy. Sadly, I have neither the skill nor the patience to actually build a beam balance of that quality (which is why I use a chemical balance or a Jolly balance).

The beam balance, when well made, is incredibly accurate and consistent in use, and, by use of suitable adjustments to the size of the riders, is capable of a broad range of measurement. The limitations to the design are the friction of the pivot and the rigidity of the beam.

There is no limit to the choice of scale used on this type of balance provided that it is linear and is aligned properly with the pivot (the pivot should be at zero units). Indeed, a scrap metal merchant I once visited had a beam balance weighbridge scaled in hundredweights - that is 4" per Ton, and was capable of handling 20 Tons of scrap. (a beam of 80 inches length that is 6ft 8" or 2m).

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
Rudy Bolona February 06, 2011 08:59PM
Thank you for the kind words! I'm a wood floor installer by trade, so working with such materials is not much of a problem for me. Once built, this balance does require a learning curve. I have learned several tricks to getting good, consistent results. It must be calibrated and made sure it's level before each measurement. The balsam wood of the beam itself is quite sensitive to changing humidities. I use a simple compass to record balanced beam distance in air and match that setting in the submerged in water reading. Very accurate this way.
Don Saathoff February 07, 2011 01:27AM
I found an old analytical balance in an old photo studio years ago. The photographer had no further use for it so I built a double pan assembly for the left side, balanced it, added a beaker for DI water and it has served me well for 20 years! Quite repeatable and since D is essentially a "dimensionless" number, weights for the right pan can be anything - using the beam rider and chain assembly gives a good four-place reading, so far repeatable to three places. The agate fulcrum & pivots (knife-edges) make friction negligable.

Don S.
Alexandr E. Zadov February 07, 2011 05:45PM
I am sorry for machine translation.

I am taken by definition of density of minerals many years.
My councils:
1. For the big crystal almost any physical method on the basis of Archimedes law allows to define easily density +/-0.1 gram/sm3. All methods simple. Choose a method under scales available for you. The scale of 0,01 gram is sufficient.
2. Small and smallest crystals demand a special technique. Here careful, laborious work and other scale of scales is necessary.
3. For primary diagnostics accuracy +/-0.1 gram/sm3 is quite sufficient.
4. Accuracy above +/-0.01 gram/sm3 for mineral definition is superfluous. (You will see, than one crystal of the same mineral differs from another. Even if crystals seem faultless. This influence of micropores and inclusion other minerals.)

Don Saathoff February 07, 2011 07:48PM
Alexander, you are, of course, correct. +/- .1 is sufficient. +/- .01 better, if repeatable. Since I do three seperate determinations over two days (wet sample!) and average results, I'm looking for repeatability affected by initial moisture and or solubles in the sample. I go to +/- .0001 simply because it's easy and is procedural in other aspects of my work (assay). I guess you could say it has become habit!! I'm addicted!!

Don S.
Alysson Rowan February 09, 2011 07:55PM
For those tiny specimens, there is nothing better than a pycnometer, though for this method a consistent analytical technique is the deciding factor on the accuracy and reproducibility. A single drop of teepol or similar wetting agent per litre of distilled water is a must for this technique, since it relies on near-perfect wetting of small samples.

My article does explain the method and, more particularly, how to make a servicable pycnometer bottle.

I have seen this method used to find the specific gravity of a single, small sand grain using a tiny (0.5 ml) pycnometer bottle.

You will require a precision dry-pan balance (Jolly or beam balances are perfectly acceptable) for this method, and the mass of the bottle is irrelevant as it is weighed dry to start with.

When you invited me to see your etchings, I didn't expect to
see so much degraded quartz. Really.
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