jgourlay
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If possible, need a quick and dirty test for lead
Gents, is there a lead and or barium test that can be performed with stuff typically laying around the house?
Here is the situation. I bought a potters wheel (used). As a bonus, it came with numerous 3 gallon buckets of "clay". This was a deceased husband
sale.
One of the buckets is labeled "white matt cone 6". There are many non-lead and non-barium based ways to make a cone 6 matt white glaze. There are
also countless barium carbonate and lead oxide recipes. The stuff could be liquid or powder, I haven't yet opened the barrel.
I'd like to know whether it's lead/barium based before using/tossing.
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Lambda-Eyde
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Dissolve in nitric acid, precipitate lead as PbCl<sub>2</sub> (poorly soluble) with NaCl. Both barium and lead can be precipitated as the
sulfates.
The question is, can this test give a false positive result? That is, does any other ions used in 6 matt white glaze precipitate on addition of
Cl<sup>-</sup> or SO<sub>4</sub><sup>2-</sup>?
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jgourlay
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Right. You never know what's in this stuff. Combinations of various aluminum silicates, feldspars, in a white possibly zinc oxide and tin oxide
(white, and matt respectively)
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matei
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To check for lead, You can also try to dissolve in nitric acid, neutralize most of the acidity with NaOH and treat it with KI. PbI2 is a yellow
precipitate which dissolves in excess of 6M KI solution due to the formation of [PbI4]-. Also you can add potassium chromate to the above neutral
solution - Pb2+ and Ba2+ will form a yellow precipitate of PbCrO4 or BaCrO4 which is insoluble in acetic acid (other chromates such as CaCrO4 or
SrCrO4 are soluble in acetic acid).
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chemrox
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there's a geochemical test for lead I will try to look up and post next week .. involves dithizone. I've used it for prospecting and environmental
investigations. I think Zn also registers so if there is Zn present you may get false positives
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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woelen
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The KI-test is quite specific for lead and it even detects the metal in small quantities. It does not give any results with barium though. If you have
both KI and K2Cr2O7 (or K2CrO4) then you could add KI to one sample and K2Cr2O7 to an other sample. If only the K2Cr2O7 leads to formation of a yellow
precipitate, then you have barium. If both the KI and K2Cr2O7 lead to a precipitate then you have lead.
Another sensitive test for lead is the addition of any sulfide (e.g. sodium sulfide, but also H2S, bubbled through the solution works). If you have
sulphur and iron powder, then you can make some FeS and add an acid to this. Bubbles of H2S are formed. Keep a paper, soaked with the sample to be
tested in the H2S gas. If there is lead in the sample, then the paper will turn dark, even at very low concentrations of lead. Again, this test does
not work for barium, it does not do anything with barium.
For all tests, mentioned above, it is necessary to neutralize most of the acid in the sample to be tested. There is no need to do this precisely, if
you dissolved the sample in nitric acid, then just add enough NaOH such that it is only weakly acidic.
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I am a fish
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Barium (and to a lesser extent lead) may be detectable using the flame test. The emissions could well be masked by other colors, but you could
probably still see them with a CD Spectrometer.
1f `/0u (4|\\| |234d 7|-|15, `/0u |234||`/ |\\|33d 70 937 0u7 /\\/\\0|23.
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unionised
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Is the bucket particularly heavy?
If not then it hasn't got that much lead or barium in it.
Also, are you planning to eat this glaze? If not, does it matter that it gas Ba or Pb in it?
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not_important
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Many barium containing glazes only have roughly 5% wt. BaCO3 in them, I've seen a few that run around 16%; this is dry ingredient weight before mixing
with water. As such that's not enough to increase the density of the glaze mix enough that you'd notice it by heft, and even using a scale isn't
useful unless you know the amount of water.
Lead based glazes run a higher percentage, maybe 15-35 % PbO; you might notice the higher density.
While HNO3 will work if the barium is present as BaCO3 or the lead as lowr oxides, it is less successful at extracting them if the barium is BaSO4,
the lead as silicates, or either metal was added as a part of a frit. For those cases fusion with a 5 to 10 X excess of soda ash is needed, followed
by disintegration of the fusion mass with cold water, and the dissolving of the solids in HNO3.
The acid solution should have sodium acetate added to neutralise all the mineral acid, raising the pH into the range of 4 to 6. In such a solution
Pb, Ba, and Sr chromates will precipitate, but not Ca; Sr make take some time to start to precipitate. It's also a good pH for precipitating PbS.
A quick test for Ba & Pb requires you to make a saturated solution of CaSO4, say by adding a couple of grams of plaster of Paris to a liter of
water and stirring it for several hours, them allowing it to settle for a couple of days. This contains very roughly a mg of CaSO4 per milliliter.
Adding it to the sodium acetate neutralised solution will result in the immediate formation of ppt of BaSO4 or PbSO4, while SrSO4 will form only
slowly if at all. No other metals likely to be in glazes interfer.
Adding KI to get yellow PbI2 is a good confirmation, excess KI can dissolve some of the PbI2, diluting reprecipitates it. The chromate test is good
for Ba, although Pb reacts as well.
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Panache
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PbSO4 can be completely separated from silica, barium and tin sulphates by extraction with hot aqueous ammonium acetate (20g AA/100ml h20). it is
important to not have any free sulphuric left in the PbSO4 as this may interfere. Basically keep extracting the solid (is dissolve in hot aqueos
AA,filter etc) until a nil test for lead is achieved, remaining (if there is any) is barium.
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not_important
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A note on the quick test using a solution of CaSO4. The amount of Ba/Pb SO4 formed is small, given the low solubility of CaSO4, so it is mainly
useful for showing that either of both metals are present; chromate will also detect Sr and Ca in higher concentrations. The amount of ppt is small
enough that separating Pb from Ba is not practical (*), it's better to go with bulk precipitation using H2SO4 or an alkali sulfate, then working up
the sulfate ppt to determine Pb and Ba in it, with Sr and Ca also ending up in it. The acetate method Panache gave is a good way to separate Pb from
Ba & Sr, some Ca will go with the Pb.
* - you can centrifuge the ppt, wash it once or twice, then add a solution of H2S or Na2S - if the ppt contains Pb it will darken and turn black.
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