Glass Message Board
Glass Identification - Post here for all ID requests => Glass => Topic started by: Springhead on January 27, 2008, 10:45:42 PM
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Anita...
That white stuff looks like Rainbow WV glass co.
Not sure it is glass... heh....
Tap it on your teeth like a pearl or if you don't care
stick a red hot pin in it... Also... plastic items like that usually will have very fine striations on the base where it was cut.
Yes... billiard balls are made of plastic...
Breaking News
I just looked at the close up of the bottom
there are the striations I mentioned
heh...
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Well, being a bit of a strange person and a former biology teacher, I decided to check to see if the dog was plastic using the method of Archimedes -- no not Seguso mispelled. I weighed the dog and dunked it in water to check its volume, then calculated the density. The density was 2.3 g/cc. Is there a plastic this dense? I thought probably rock, so I put a lit match under it. Nothing except some soot up the side. Conclusion: probably carved rock or something similarly dense and inorganic.
As an add-on, I did this for something I knew was solid glass and got a density of 2.8 g/cc. Yes, I know... I need a hobby.
Anita
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Perhaps it's a type of resin? There's some marvellous sculptures in resin these days.
Snowflake obsidian would be the closest 'rock' I suppose, but then it'd have to be black and white all the way through...not just the edges I should think... :)
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Or maybe sandstone or concrete. That would be in the right density range, given a margin of error for my crude measurement technique.
I checked on the density of some things. I discovered that my solid glass piece was denser than many types of glass. It was a jade glass elephant. Whatever was added must have been pretty heavy.
There is a huge range in the density of glasses. I wonder if density could be used as a diagnostic to tell the different types of glass, e.g. Chinese from Murano or Salviati from Seguso. I'll have to test a few bubble-free pieces.
Anita
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This might be a bit off-original-topic but in response to Anita's comment about whether glass density could be used for Id ...
... that has been attempted over many years (at least from 1969 to my knowledge) for paperweights. Although density alone can sometimes reveal interesting points, and can in some cases assist with Id to a possible maker, it is not an exact science. In most caes, the best that can be deduced is that an item may or may not have a lead-based mix.
This is a complex discusion and perhaps a Moderator could split this out as a new topic. We could then see what other folk think or know from experience of trying density measures. Also we could learn how different folk go about it and assess whether anyone's particular method could be regarded as a "standard" that anybody could use to achieve acceptably similar results. (Some folk already know my views on that one.)
I, for one, have not found it easy to replicate measures even for my own items - and in one case, where a US friend mesured the density of a paperweight then shipped it to me, I measured it several times and never managed to match his reading! And has anyone actually achieved an accurate density measure of, say, a 10 inch high vase using home-based equipment??
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Before it gets split off, I'll add another thought. The density of glass can depend so much on what is in it and how much. For example, leaded glass is heavier than non-leaded. Perhaps density can be used to tell if a piece is a copy if the technique is used on both. For example, if someone measured the density of Seguso's 4.5" black & white hound dog, it may be so different from my dog's density that it would indicate that something was off.
Of course, those striations on the bottom of my dog will tell us that, anyway. I guess I could say it was a dog done by Scarpa for Venini with an inciso base. ;D (Just kidding here)
Anyway... I guess the take-home message is to ask to see the base of a B&W Seguso dog if buying online. A seller might think it strange if someone asked for a density measurement! I think Svazzo has one of the Seguso dogs. I'll have to check his site.
Anita
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Of course, those striations on the bottom of my dog will tell us that, anyway. I guess I could say it was a dog done by Scarpa for Venini with an inciso base. ;D (Just kidding here)
thanks for my morning laugh, Anita ;) "inciso base" hee hee hee :clap:
bidda
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There is an detailed description of measuring specific gravity in Paperweights forum.
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Yes, there certainly is an SG measure discussion in Paperweights. It starts at this point (http://www.glassmessages.com/index.php/topic,13465.msg87995.html#msg87995) in a message about a Clichy weight. In that discussion I never got around to adding to my comment, "I have a couple of questions, but I need to think about something first".
One point was about methods of allowing for variations due to differing water temperaures which can have a critical effect in some cases. Alan had mentioned that the paperweight being measured should be the same temperature as the water. Perhaps that does work, but I have not yet tested it for myself. But it raises another question or two. How is the temperature of the item measured? Or is it enough to place the item in the water for a length of time such that the temperature equalises - and if so, how long is enough time?
Another point was about the method of measuring the volume of the target item by using an "overlfow method" and weighing the water spilled from an absolutely full cylinder. In this case, how is the effect of surface tension overcome? With a cylinder wide enough to be able to dunk an object larger than about 2 inches wide, the amount of water held at the rim by surface tension can be surprisingly large. And it can be large enough to seriously affect the calculations. A cubic centimeter of water is not that much when spread out and held by surface tension, and 1cc of water will have an effect on the decimal places of the results.
And all that was to do with paperweights. What about other glass objects? There was a discussion about this a couple of years ago in The Glass Cone (the Glass Association publication) and it seemed to me (a long time after I first read the exchanges) that the person favouring the "volume displaced" method must either have had a very finely marked measuring cylinder - not generally availble in DIY stores - or they used the "fill up and overflow" method, and since they had not mentioned it, it appeared that they had probably not accounted for surface tension.
I experimented with a tip I got from somewhere (can't now recall where) and used a drop of washing up liquid to overcome surface tension, but even so, a fairly substantial bulge of water can still be retained at the top of a full cylinder (or a kitchen measuring jug!) after the overlow has stopped.
When using the "weight of item in water" method, there is a possible problem with whatever is used to suspend the object in the water so that it does not touch the base or side of the cylinder, bucket etc. If an absorbant material is used, such as with some types of "net bags", then it is possible that enough water can be absorbed up the material such that the weight of the material outside the main water compromises the calcualtions.
Perhaps my detailed concerns are not actually relevant in that my own experiments have been flawed and there really is no actual compromise of the measurements. Can anyone here confirm they have found those sort of concerns to be red herrings?
In any event, I am still interested to learn how other people may have made specific gravity measures of larger glass items. And can SG measures be accurately repeated by people using different equipment? In theory, different equipment and the two main methods ("amount of water displaced" or "weight of item immersed") should not matter, as long as the baisc method is properly followed and measurements are carefully recorded. And of course, all of us attempting SG measures always ensure that there is no air whatsoever trapped within or beneath the object when it is supsended or "dropped" in the water - or do we?. But if there are differences, then it leaves any shared analysis by several people open to doubt.
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Surely the better way to measure the density is to use an over-large measuring vessel, graduated on the side, and only fill it halfway (for example) with enough volume left for the item being measured. That way you measure the difference between two levels. The overflow method is a quick method, but could be rather imprecise I would have thought.
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Suitable laboratory measuring cylinders/beakers sell on eBay quite cheaply, ones large enough to immerse a 10 inch vase not so often but they do turn up occasionally with huge postage costs. I skipped buying one on eBay as the 15kg postage made it too expensive.
A simple solution to the meniscus problem, for large vases etc, fill any container (any material) large enough until the meniscus is proud of the rim and then spoon enough out water into the object, as the level goes down a larger scoop can be used until it is possible to fill the vase and then using a wire holder lower the full vase slowly in to the container until immersed and the water displaced over the rim of the container. Now add sufficient water to bring the level up again to the starting height again. Slowly remove the vase. Now refill the container with measured amount of water and you will have the volume.
To decrease water loss in the process one drop of a wetting agent (washing liquid is ok but avoid turbulence) to reduce loss from water sticking to the scoops and the vase on removal. This should give a reasonably accurate measurement. Temperature balance is important but just leaving the water and item in the same place without drafts for an hour or two should be sufficient.
It is probably best to use distilled water too.
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Distilled water made absolutely no difference to my own tests. I found that there was no noticeable change to results from those using ordinary tap water (which in my chalk-ground location is a hard water).
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10" high 7" diameter 4 litres http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=190194572696
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Phew, pity this is in Birmingham, Alabama... not England :-\
But my only reservation is the 250ml graduations, which makes this too imprecise. There is undoubtably a more precision vessel with smaller graduations, but probably very expensive.
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You need to measure between the graduations and calculate the in betweens, equally accurate as what ever you use to measure. Vernier calliper can give you accuracy of about .1 mm.
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I wrote an article for the PCA Bulletin a couple of years ago which covered in passing some of the issues in this thread about measuring density. I have set out an expanded version below. My belief is that for anyone trying this at home, the 'weight in air and weight in water' method is easier and more accurate than measuring the mass and the volume separately.
Density and Specific Gravity, and the accuracy of measurements
Specific Gravity (SG) is a measure of how the weight of something compares to the weight of exactly the same volume of water, and is a pure number. Density is a measure of the mass of a substance in a given volume, measured for example in grammes per cubic centimetre (g/cc). Because the density of water at 4 deg C is 1.000 g/cc, the numeric value of density and specific gravity is the same at 4 deg C. So we do not really need to worry too much about the difference when talking about paperweights. It is easier to type or say ‘density’ than ‘specific gravity’ !
The commonest method of measuring the SG of paperweights is to measure the weight in air, and then the weight when immersed in water. This allows us to calculate the SG: the apparent change in weight is due to the upthrust from the displaced fluid, and is equal to the weight of the same volume of water as the paperweight. There is a very small upthrust from the air when measuring the weight in air, but this is tiny, and we can ignore it. This method has the advantage that the measuring scales need not be accurate, provided they are linear and correctly zeroed! Linear means that if you weight two laboratory 1000g weights and get 1000g for each, you ought to get 2000g for the pair weighed together. The reason that the absolute value does not matter is that the calculation depends upon the ratio of two measurements - so they can be eg 2% wrong, and it has no effect on the calculated SG. The absolute method - ie weighing the item in air and determining the volume by displacement of liquid - is fine theoretically, but requires an absolute measurement in each case. The scales must be spot on - if they read 2 % wrong, then your density will be 2% wrong. And the difficulty of making an accurate determination of the volume has been rehearsed earlier in this thread - it is fraught with inaccuracies.
It is not difficult to overcome some of the difficulties described for the immersion method. I use a rod fixed to a pivot at one end, resting part way along on a knife edge on a pair of electronic scales. The item is suspended in a cradle of plastic coated garden wire at the free end. The reading on the scales is adjusted by moving the knife edge so that the highest reading possible is obtained in air - that way the errors are reduced. The plastic coated wire does not absorb water. And when the apparatus is ready to go, you 'tare' the scales to zero. Then add the item - you get a 'weight in air reading (A)'. The bring a jug of water up from below to immerse the item - you get a 'weight in water (W)' reading. The SG is calculated as A/A-W. eg A=500, W=350, SG = 3.33. If the item is the same density as water, then W will be zero, and you get SG =1.00. Note that units are cancelled out, and absolute measurements are not required - suject to the caveats above.
What about the effect of water temperature? At 20 deg C, the density of pure water has reduced to 0.9982 g/cc, because water expands as it gets warmer. So too does the glass of a paperweight, but water expands more rapidly than glass at temperatures around 20 deg C. So, for example, if a paperweight and a jug of water are warmed from 20 deg C to 25 deg C, the water will expand more than the glass, and the density of the water will decrease more. A paperweight at 25 deg C immersed in water at 25 deg C displaces slightly more water (the paperweight increases in volume by around 135 parts per million) but because the water is less dense, the upthrust from the water decreases by 1050 parts per million. So the apparent weight in water is greater, leading to a higher calculated specific gravity figure (915 parts per million) – roughly one part in a thousand for a 5 deg C change in temperature. But as long as we know the water temperature we can adjust our measurements – and anyway, a couple of degrees will not make a significant difference.
What about the purity of the water? Water does vary in density depending what salts are dissolved in it, but the difference between distilled water and typical tap water is very small – about 30 parts per million at room temperature, and can be ignored for the purposes of this article.
Measurement accuracy. This is very important. A typical Old English paperweight weighs about 500g, and has a density of about 3 g/cc. The apparent weight when submerged in water will be about 330g. Provided we take care to minimise systematic errors, then the greatest accuracy we might achieve in an individual measurement using non-specialised equipment is a tenth of one gram: if the true SG were 3.000, our measured value could then vary between 2.997 and 3.003. That is about plus or minus one part in a thousand – similar to a 5 deg C change in water temperature. If our accuracy is no better than about half a gram, our measured value would vary between 2.985 and 3.015. That is about plus or minus 5 parts in a thousand, and is far larger than any of the other factors discussed above. This range can be reduced somewhat by taking the mean of several repeated measurements, but accuracy and reproducibility are the most important factors when measuring SG. Indeed, repeating measurements from time to time on the same weight is a good test of the reproducibility of the measurement system, and I have done this with satisfactory results.
Alan
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Interesting method. Took me a while to work through the physics of it, it's been so long. You're measuring buoyancy to find specific gravity, it sounds like - is that right? What about the buoyancy of the wire net, do you account for that somehow?
Thanks for sharing your article with us. Very informative. I don't suppose you have a photo of the system on hand, do you?
For the "old fashioned" way of measuring water displacement, as long as the vessel containing the water is relatively straight-sided, measuring from the top or the bottom of the meniscus, as long as one is consistent about it, would be more precise than trying to measure volume by filling the vessel to the top and having to deal with surface tension. One of the few things I remember from chemistry class.
Another thing you can do if the vessel is cylindrical is calculate the volume of water displaced, rather than try to measure it. Volume = Pi X radius2 X height, where height is the difference between the height of the water column before and after the glass is added (a good use of those vernier calipers!). A cylindrical plastic juice pitcher is big enough for many types of glass items.
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As I am still awake in the early hours, I will give a response to a couple of things, but maybe Alan (and Derek) will add their own comments later.
What about the buoyancy of the wire net, do you account for that somehow?
The "net" that Alan uses is actually a minimal setup with just a couple of loops of the plastic-coated wire. It therefore has negligible effect on the bouyancy.
In my own experience, I prefer a more substantail "net" but that does have a danger of affecting the measures if there is a lot of material. However, another paperweight collector who has also measued SG (density) has given me a sample of what she has used and claims no problem - it's a ladies' stocking sock, which does not absorb water, is of minimal weight and gives a full support of the item being measured. I have yet to try it for myself though (been meaning to for at least a year!!).
Of course, whatever "net" is used, the balance has to be "tared" first (as Alan has said) with the "net" in place. The main issue with bouyancy is to ensure that no air is trapped beneath or within the object being measured, or within any links in the "net", when it is set in the water.
I don't suppose you have a photo of the system on hand, do you?
A simialr system is used by Derek, who also posts in this board. He showed a photo of the system in another message, but it was a tinypic image and has now disappeared. Perhaps Derek can resurrect it for us? In the meantime, the message, which veered off into an earlier discussion of SG measuring may be worth looking at:
http://www.glassmessages.com/index.php/topic,13465.msg87635.html#msg87635
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Oh boy,
KevH in stockings.......... :o :o :o
Marcus in (birthday) suit being relatively dense???????
we could have some serious (not/maybe) fun here.........
Anne, GMB calendar 2010 suggestions ?? ;D ;D
M
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Hey, I'm all for Marcus in his birthday suit - go Marcus! Go Kev! Show dem legs...:hiclp: :thup::hiclp: :hiclp: :chky:
The main issue with bouyancy is to ensure that no air is trapped beneath or within the object being measured, or within any links in the "net", when it is set in the water.
But what if your net acts like a psuedo-bubble? The specific gravity of something like a stocking would be lower than glass, right? Even if you tared out its weight, it's still a part of the equation because it won't stay constant when immersed. You could get around that by measuring the specific gravity of the stocking...or could you? Wouldn't it float? ;D Hmm, maybe if it was fishnet...
This is all kind of hypothetical. I don't know what degree of error is acceptible for these things, or whether that might make a difference. Now there's a thing to test - effects of different holders.
I still can't visualize the set-up. I'm bad at that kind of thing.
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Here are a couple of images of my 'high tec' set up. It is in practice accurate and consistent.
a. A 'knife edge' support which can be moved along (as can the scales) so the scales read near their maximum. But don't move it during a set of measurements! The knife edge may look like a piece of meccano - which it is - but it has a sharp ridge on the top. The scales read to 1 gram, and up to 2000g, so I try to have readings in the 1900 - 2000g range. That way the error is 1 in 2000, not 1 in 300 or whatever. And I do other things to improve accuracy, such as repeat measurements.
b. A rigidly fixed pivot.
c. The paperweight suspended in a wire cage. Pontil upwards so it does not trap air. The item is first weighed in air above the water, then the water is raised around it - so no part of the measuring system is moved. I raise the jug and stand it on a support, making sure that the paperweight is not touching the sides or base. If necessary I pour in a little additional water.
Regarding upthrust on the wire cage - yes, there must be a very small effect, but too small for me to measure. You can see from the image that very little wire will be under water.
Alan
(http://i175.photobucket.com/albums/w155/rosismum/SGequipment.jpg) (http://i175.photobucket.com/albums/w155/rosismum/SGequipment2.jpg)
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Thanks, Alan! Now I can see it! And you're right, there's not much wire at all that gets immersed. Just out of curiosity, have you (or has anyone) compared measurements using your system vs. the water displacement one using the same 'weights? It would be interesting to know whether there are consistent differences in the two ways of finding density if two systems are commonly being used to measure them.
Oh, re: the stocking question...just saw this again from Kev's post, "If an absorbant material is used, such as with some types of "net bags", then it is possible that enough water can be absorbed up the material such that the weight of the material outside the main water compromises the calcualtions." That's another good point, the capillary action of whatever's used to hold the things. The few strands of wire definitely seems a better option!
I wonder how much altitude affects measurements of a weight in water. Would someone in Denver get the same reading as someone in Denmark?
(Yes, I'm a frustrated scientist, inflicting myself on the GMB.)