M Richens on fri 11 apr 97
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In article =3C334AFA89.6656=40glass.com=3E, =22Karl P. Platt=22 =
=3Ckplatt=40glass.com=3E
writes
=3E----------------------------Original message----------------------------
=3ELet's start this by saying that of fundamental importance here is that
=3Ethose among us who are in tableware want to have the hippest glaze at no
=3Eexpense to long term durability. I will submit that essentially all
=3Epotters in the business understand this at this point and am not going
=3Eto proceed on the premesis that they (we) are idiots.
Then why talk down to people?
Let me stress that I feel that in a properly formulated AND fused glaze,
which has been tested properly to confirm its state, a level of lead and
barium which does not leach excessively is no bad thing commercially
(even 1=25 Lead as LBS in a glass composition will produce a much nicer
glass). But while there is an element of doubt about the durability of
the glaze then you must err on the side of caution. As I say.. test it.
Compare European (British) lead release standards to FDA and then
Proposition 65
=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A==
2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2=
A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A
Bear in mind that Commercial china especially where there is a US market
will be tested to meet FDA criteria for ALL production.
=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A==
2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2=
A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A=2A
BS6748 FDA Proposition 65
Flat approx 8ppm 3ppm 0.226ppm
Holloware 4 2 0.1
cups 4 0.5 0.1
Jugs 4 0.5 0.1
There are other restrictions on the size of the sample, the number of
units tested and the criteria for assessing failure conditions.
There are strong opinions that the levels of release covered by the
British Standard will not constitute a hazard to health (There are
arguments the other way as well)
AND
that the health benefits of having an easily cleanable,inexpensive,
easily available product for food preparation and delivery
(A plate :-=7D)
far outweigh the other hazards.
If however the FDA adopt Proposition 65 as a US national standard then
it is more commercial sense to eliminate lead altogether than to label
everthing as 'This product contains Lead'.
This is obvious. It is also within the realms of probability that focus
of pressure will switch elsewhere. The likely targets =5Fwhether the
potter (and remember that even the big companies are potters at base
even though they take a more engineered view and may not admit it)
likes it or no=5F will be Barium, Bismuth, Strontium.
I may be a cynic but in I feel that in certain areas the level of
release allowed is inversely proportional to the sensitivity of the
equipment available to test it regarless of risk levels. (Max's theory)
The corollary does =5Fnot=5F hold (If you don't look for it, it doesn't
count)
Now Barium has a lot of good properties. It also has a LOT of compounds
which start at harmful and go all the way up to scheduled poisons.
The packing requirements for transporting Barium carbonate over water
are fairly tight due to the effects on fish.
=3ENow,.......
=3E
=3E=3E=3E True but who is saying the Barium is released by diffusion? If the=
Al
=3ECa
=3E=3E=3EMg are attacked and chelated the Ba++ will fall out. This should =
not
=3E=3E=3Ehappen with a properly formulated =5FAND=5F fused glaze but....
=3E
=3EThis scenario is largely implausible. Let's go on to the net item.
You may think so. Try talking to a chemist. or a good glass scientist
(Put a tooth in a glass of Coke and watch it go.)
=3E
=3E=3E=3EWhat are you talking about: glaze or crystal?=3C=3C
=3E
=3EIn the sense of crystal glass or crystalline materials?
=3E
Don't fog. you started talking about Barium Silicate crystal bond
strength.
=3E=3E=3EWhy would it be BaSiO3?=3C=3C
=3E
=3EBecause carefully taken mesurements of the differences in thermodynamic
=3Eproperties of these reactions have been made and show this to be the
=3Ecase. See A. Paul., Chemistry of Glasses, Chapman and Hall.
=3E
I'm sorry but why not Barium Aluminate or
Barium(0.1) Calcium(0.9) Silicate or any combination..BUT we were
talking about glasses weren't we.
Barium may be a big element but the thing to remember in the glass
'structure' is that it sits in a big hole.
=3EIn the case of a pure BaO-SiO2 melt, which is difficult to produce in a
=3Ehomogeneous form owing to immiscibility, BaO does, indeed serve to
=3Edisrupt Si-O-Si bonds. However, in the presence of Alkali it's behavior
=3Eis fundamentally different. Ba (Mg,Ca,Sr,etc) have the effect of
=3E=5Fstabilizing=5F the melt rendering it more stable than it would have =
been
=3Ewithout the addition.
That was, indeed, partly correct. They lock in the =5Fdisordered=5F
-S-O-Si-O- structure preventing it crystallizing or devitrifying
(however you want to look at it)
=3E
=3E=3E=3EpH is =5FNOT=5F the only criteria. Citrates can complex Al+++ and =
make it
=3Esoluble so why should Ba be different.=3C=3C
=3E
=3EWho said it was? But it is and the substance of that discussion my be
=3Emore appropriate elsewhere. The use of the concept of pH has been very
=3Ecavalier here.
Then why did =5FYOU=5F treat it that way...in isolation.
=3EI simply stated a series of facts for a clearly defined
=3Eset of circumstances.
but not realistic ones.
=3E Besides, in the real world, even if these
=3Ereactions do proceed, their rates are extremely slow againt the well
=3Emade glaze. We're not discussion the pace of reactions one might see
=3Ewhen they light a match -- or a stick of dynamite.
I have not disagreed about WELL MADE GLAZE.
(Total Irrelevancy possibly But remember that the Arsenic which was
found in Napoleans body came from the wallpaper of the house in which he
was confined. Lovely green colour that.....)
=3E
=3E=3E=3EIt is not just down to volume of liquid. A small volume spread over=
a
=3Elarge surface area can produce more attack than a large volume due to
=3Echanges in the state of the solution and attack mechanisms.=3C=3C
=3E
=3EI'm not sure what this is getting at. We're talking about a cup, not a
=3Etile floor. Assumng evaporation doesn't take place it remains that a
=3Egiven volume of solvent can only dissolve =22x=22 amount of a substance =
in
=3Eany event. If it is spread thinly over a wide area the net amount of
=3Ecorrosion (leaching) which might occur in a given area is significantly
=3Esmaller than in the case where it is confined in a cup.
Not so. The point I was making is that in a small amount of liquid the
changes to its chemical composition are such that the type of attack on
the glass surface changes and thus changes the glass surface.
Look up Hench on the types of leached surface layer in glass (1977) I
don't have the reference to hand. But he's a Professor at Florida State.
=3E
=3E=3E=3EWho says they wash the cup between each drink?=3C=3C
=3E
=3EWho cares. In this case there's probably more risk from bacteria than
=3Ethe glaze.
Not You Obviously=21=21
=3E
=3E=3E=3EYes and increase surface area for attack.. and =5Fpossibly=5F =
produce two
=3E=3E=3Ephase glaze with one phase much more soluble than the other..
=3E
=3EThe odds of having two phases in a ceramic where one is more soluble
=3Ethan the other in a given medium are very high -- even if these are,
=3Esay, ZrO2 and Al2O3. It is the relative solubility that we are concerned
=3Ewith and this should be elaborated.
=3E
=3E=3E=3EI also found that using =3E 3=25 Barium in a glass.. (Remember =
everything
=3Eis
=3E=3E=3Efritted together first then milled and applied).. would sometimes =
give
=3E
=3E=3E=3Eenamels which were unstable in slip form due to Ba leaching.
=3E
=3EThis doesn't mention that the enamel frit likely contained up to 8 or 10
=3Emol=25 B2O3 in combination with the BaO. Let's be clear that enamels =
are,
=3Eby definition, substantially weaker in structure than most all
=3Ecommercial glasses and ceramic glazes and it is unreasonable to draw
=3Ecomparisons between the two.
Don't put down things you don't understand. The statement you make is
nonsense. The company I was R=26D manager of was no Ferro but we had a
=A32,000,000+ (=243,000,000) TurnOver and were profitable.
Some of the enamels I developed would withstand molten Aluminium for a
lot longer than the cast iron which they coated. We produced enamels
which lined what are known in the trade as Glass Lined chemical reactor
Vessels. We developed and produced enamels which would take electric
elements for flat heaters. Our products coat and enhance domestic
appliances in the UK and Europe. These are the enamels I deal with and
understand best.
Consider Ferro or Mobay-Pemco even. They produce the range from glass
linings and farmyard silos to jewellery enamels to enamels for aluminium
saucepans to glass enamels to Enamels which will withstand =5Fregular=5F
heating up to 600 degrees Celcius on the pyrolytic ovens.
Why don't you make the same statement to one of their ceramic engineers
involved in enamel. (the response may not be as mild as mine)
=3E
=3EAnyway, this is all to show that there's a great deal occuring within
=3Eceramic glazes that are worth considering in more detail here. And,
=3Eagain, I am urging this. However, I will remain firm in the conviction
=3Ethat the grand majority of tableware glazes which contain BaO are
=3Eperfectly safe to use
I agree.. if properly formulated and fused.
=3E -- remember, there are =5Fno=5F leaching standards for
=3EBa reflecting either that there are no problems and/or that the no-one
=3Ehas yet managed to create one. Show me one potter or a customer of
=3Etheirs in the recent history of potting laid low by Ba and I will
=3Eshut-up and go away.
Yet....
Show me the bodies=3C?=3E
=3EI will also cite myself (again) as a living
=3Eexample of someone who has been working with these =22deadly=22substances=
in
=3Equantity, eaten off of Ba blue plates, used Pb glazed plates my whole
=3Elife (so have you), enjoyed my Pb glass stemware and so on since the
=3Etime when I had no gray hairs ( but I ain't old) and have suffered no
=3Emaladies as are gravely predicted here....and there. Guys, if anyone is
=3Ethe guinea pig for all of this dire spooge it's me and I am not a
=3Emoon-suit sort of guy. Tidy, yes. Paranoid - why bother.
There is some paranoia and panic (two of the =5Famended=5Fnumber=5Fof=5F
Horsemen of the Apocalyse)
=3EI'd rather
=3Eenjoy the sunrise over Lagoa da Conceicao instead and reckon I'll be
=3Edoing that for another 40 years or so. I'd suggest it to you, too
=3E
put a chair out with a gin and tonic.
=3ERegards,
=3E
=3EKPP
Continued on next rock....?
Max
--
Max Richens max=40richens.demon.co.uk +44 (0) 1925756241
Enamel Consultant - Ceramist - Analyst programmer
Software for Batch Formulation and Millroom control.
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