Topic: Testing densities (split from Re: Black & White Hound Dog)

[Springhead]

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...

[TxSilver]

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

[Max]

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... 

[TxSilver]

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

[KevinH]

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??

[TxSilver]

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.  (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

[bidda]

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.  (Just kidding here)

thanks for my morning laugh, Anita "inciso base" hee hee hee  :clap:

bidda

[Frank]

There is an detailed description of measuring specific gravity in Paperweights forum.

[KevinH]

Yes, there certainly is an SG measure discussion in Paperweights. It starts at this point 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.

[David E]

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.