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.