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Author Topic: lead glass  (Read 15033 times)

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Offline krsilber

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Re: lead glass
« Reply #30 on: December 18, 2008, 02:08:44 AM »
Not sure what the date of this is, but it's after Francis's paper.  Interesting bit about furnace technology playing a role.

Quote
Ravenscroft is regarded as the inventor due to repetition in glass books that did not look deeper

I think this is true to some extent.  The books oversimplify the picture by using "invented" or "discovered" to describe his work.  Still, I'm inclined to think that Ravenscroft did something new that made his glass particularly successful, and the formula the basis of much lead glass after that.  But maybe not - perhaps the purity of materials or some technological factor played just as important a role, and he just lucked out. 

Kev, I did actually notice what you said about stability a few posts ago.  I don't know why I didn't just accept that as the key.  Apparently crizzling was a problem in previous glasses.  There's also the bit from Powell's book about "successive tentative experiments to make a more readily fusible glass."  The paper in the link talks about color issues.  So I guess there were a few problems to be worked out.
Kristi


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Offline KevinH

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Re: lead glass
« Reply #31 on: December 18, 2008, 04:35:41 PM »
All of what Kristi has surmised in the post immediately above is true to some degree in the complex issue of what Ravenscroft did, and why. I have had another look at the articles in The Glass Cirlce 2 and I am also reviewing again the details that R. J. Charleston gave in his 1984 book English Glass .... I will also re-read various sections of the 2006 edition of The Society of Glass Technology book on the 17th century writings of Neri / Merritt.

For clarification, I will then attempt a brief summary of things, based on those sources, hopefully including relevant points about what was in the glass batches at the time.

KevinH

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Offline oldglassman

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Re: lead glass
« Reply #32 on: April 30, 2009, 10:49:28 AM »
Hi Being new to the glass message board and having a look at previous posts ,I think the basic answer to this initial post was lost in the discussion of lead glass and its origins ,today the most reliable method of determining the presence of lead in a 17th or 18th century  clear glass vessel is to use an ultra violet lamp (blacklight),soda glass will fluoresce yellowish,sometimes very vividly , lead glass will show as bluey grey, the higher the lead content they more bluey/grey it will look ,before UV was available ,and with lead content of early glass being a determining factor of value and origin ,hydrofluoric acid was used , (not recommended today , very dangerous stuff).a small drop was placed on the pontil area and then quickly neutralized and washed off , the mark left by the reaction to the acid by the glass if white indicated soda glass and if Black indicated lead glass,thankfully UV is much easier and less deadly to use .

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Offline KevinH

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Re: lead glass
« Reply #33 on: April 30, 2009, 02:44:06 PM »
Peter (oldglassman) has made a good point about the basic answer to the original question perhaps getting lost in the detailed discusssion in this thread. The question, of course, was simply; "Can anyone tell me an easy way to tell lead crystal glass from regular glass?"

Peter's observation regarding the presence of lead in 17th & 18th century glasses confirms my own observations with the very small collection of drinking glasses I have and which I believe are 18th century.

However, for lots of items after the 18th century, the uv reactions can vary considerably. Especially for mid 20th century to modern day glass the longwave uv reaction may not show as any shade of blue at all, therefore raising doubts about the presence of lead. But checking the same items under shortwave uv will invariably show a clear blue reaction if lead was present in the mix.

An interesting point about Whitefriars paperweights (but not those made after Caithness bought the rights etc.), and probably, therefore, other Whitefriars glassware from the same time, is that they show a blue uv reaction under both the longwave and shortwave. Most other items I have checked from a similar period show the blue reaction only under the shortwave, with the longwave uv usually giving a weak (or "watery") orange / pink / lavendar colour.

UV tests are very useful, but only in conjunction with other knowledge and uv alone should not be used as a 'simple answer' to the basic question of how to tell lead crystal from ordinary glass, especially across the centuries of glass production.
KevinH

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Offline Lustrousstone

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Re: lead glass
« Reply #34 on: April 30, 2009, 03:44:47 PM »
In recent weeks, I have picked up two Kosta Boda Snowball votive (designed by Ann Warff). The other day I was randomly waving my UV light over my glass only to discover one had a nice blue glow and the other nothing. I weighed the glowing one and discovered it weighed 100g more than the other. I checked my lead crystal thoughts by more light waving in the glass cupboard. My conclusion was that the glowing one was older based on the thought that less lead is cheaper.

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Offline krsilber

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Re: lead glass
« Reply #35 on: April 30, 2009, 11:37:22 PM »
Peter, were you using a shortwave or longwave UV light?

Quote
But checking the same items under shortwave uv will invariably show a clear blue reaction if lead was present in the mix.
Kev, is this the case with all lead glass, or just from mid-20th C onwards?  Checking with SWUV (if one happens to have one) might be a good indicator.

I don't know about 17th and 18th C glass, but agree that in general there can be great variability in color under LWUV.  I have lead glass from about 1890-1925 that shows yellow, lime, light apple green, blue, grey or no response under LWUV, and have heard of pinkish purple examples.  Among American cut glass collectors the color is used as a clue to whether a piece (or at least the blank) is American Brilliant Period; those that aren't yellowish green are often thought to be European, though there is debate about this.
Kristi


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Offline oldglassman

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Re: lead glass
« Reply #36 on: May 01, 2009, 08:51:32 AM »
Hi ,
       As far as I am aware , its shortwave UVA, more commonly used for checking bank notes etc,i have no experience in testing post 1800 glass but find it very reliable for 17thc and 18thc , till the batteries run down a little then everything glows yellow,

cheers ,
           Peter.

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Offline KevinH

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Re: lead glass
« Reply #37 on: May 01, 2009, 11:49:33 AM »
Quote
Kev, is this the case with all lead glass, or just from mid-20th C onwards?  Checking with SWUV (if one happens to have one) might be a good indicator.
The short answer is, "I don't know". But a technical booklet from the 1950s (I can't remember the title right now, but a search through the Board for "UV" might unearth it) had a chapter on UV testing of glass and mentioned that shortwave uv gave a blue reaction for glass with lead. It also said that if the proportion of lead was increased to a [very] high level then the blue reaction disappeared - but I imagine the level for that was higher than would be found in the regular glass we are talking about in this Board.

I suspect that Peter's uv light is actually longwave.

My uv lamp is a "Mineral Lamp" [from the UVP company in California) with both longwave and shortwave bulbs. (There is now also version with a medium wavelength.) I think that the vast majority of single-bulb units, especially battery powered ones, will be longwave ("blacklight") ones.
KevinH

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Offline krsilber

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Re: lead glass
« Reply #38 on: May 01, 2009, 04:42:35 PM »
This is an interesting article about fluorescence of different minerals in glass:
http://www.cbrain.mistral.co.uk/home.htm
"The blue fluorescence of lead glass under ‘Hg’ UV lamps is one of the most commonly known responses."  However, the paper also suggests how hard it can be to interpret results based on a test by a single light of any sort.  For example, iron absorbs light at some spectra, and manganese may emit or absorb light (or neither) depending on its valency and other glass constituents.  The fluorescence of lead is more complex than shown here; the paper in the link in the paragraph about lead goes into more depth.  One thing that paper suggests is that lead may show different emission peaks depending on how it was added to the batch.  It also says that components may show different colors depending on whether they are joined with other atoms or not.  The take-home message seems to be that determining the content of glass through emission spectra is in its infancy, and certainly not for amateurs.

One illuminating line from the second paper: "lead, calcium, antimony, chromium, cobalt, boron potash and soda can be detected directly with this system [GABEL:  Glass Analysis By Emitted Light] and iron, aluminium, manganese, magnesium and phosphorous can be detected indirectly due to absorption effects."  Note that the graphs in the first paper suggest most of these were detected by shortwave UV, and some of them had non-visible emission spectra.
Kristi


"The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science."

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Offline David W

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Re: lead glass
« Reply #39 on: February 19, 2010, 04:18:01 PM »

Sorry to be late on the scene but this query contains two topics suitable for the glass Olympics, determining lead in glass and the question of Ravenscroft’s discovery?

1. Determining lead.
My experience over some 40 years is that blue fluorescence in the far UV is a virtually infallible way of establishing lead in glass.
I have no evidence that the intensity of colour relates to the amount of lead in the glass. The only sealed Ravenscroft I have been able to test gave a much deeper (Oxford) blue than the more Cambridge shade that  is usually seen.
With a near UV lamp (the cheap sort without a filter) lead glass may give a pure light blue,  a  pinkish blue or even a yellow/green colour with what appear to be 19th century specimens. The yellow/green colour (to my eye) is normally associated with a non-lead glass (usually soda) containing manganese as a decoloriser.
A couple of mid-19th century fairly heavy sugar bowls with a partly matted surface (probably of Manchester origin) ring like lead glass but give a whitish fluorescence in the UV. I do not know why this is.
Soda-type glasses (usually bottles and poor quality glass) often give no fluorescence probably due to the absence of a decoloriser.
Borosilicate glass may give a pink colour but I have not studied this.


All the above relates to essentially colourless glasses. Coloured glasses often quench fluorescence which makes the method impracticable.

2. How much lead?
The simplest way of determining the amount of lead in the glass is density measurement.
With a balance sensitive to 0.1% (or better) of the weight of the object,  weigh it in air and then suspended by a fine thread (e.g. button twist) in water. (put something soft in the bottom of the bucket in case the thread breaks!)
 Density =  wt. in air/wt. in air - wt. in water.  Unless ultimate accuracy is required corrections for temperature difference can be ignored but aim for 20 degrees C.
Some 60 or so 18th century English glasses I have measured all had densities of above 2.9 or at least 30% lead in the glass. Non-lead glass has a density below 2.5 and demi-crystal about 2.7 – 2.8 although I have not made many measurements on this. My  work on 18th century English glass is published in Paper no. 162 (1973) of The Glass Circle entitled Understanding the Colour of Old Glass. See also E.M. Elvillle (1951) English Table Glass, Pub. Country Life.

Later barium glasses may give high density values but will not fluoresce blue.

I hope this is of some help.

The Ravenscroft question I will deal with separately.

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