Note: Descriptions are shown in the official language in which they were submitted.
~19~
This invention relates to a process for treating the surface of a solid article made
of a vitreous materiai and containing some amount of heavy metals, said surface
being liable to come into contact with products of acid characteristics, in order to
prevent said heavy metals from drifting into said products.
5 These "articles made of vitreous material" refer to the lead glasses, the crystal
and similar materials used under solid shape, i e having no specific superficialcharacteristics; "heavy metals" refer to metals such as lead, baryum, cadmium and
the same, which can be dangerous for human health if they come into contact for a
long time with food or beverages; "products of acid characteristics" refer to alcohols
10 such as cognac, whisky, wines as well as to fruit juices and aggressive pasty products like mustards and to any other acid foodstuffs.
As a detailed illustrative example of the process of the invention, the application
of this process will be now described to the treatment of lead glass or crystal flasks
designed to be filled with alcohols in order to prevent the drift or migration of the lead
15 contained in the internal surface of said flasks into said alcohols.
It should be understood that the term ~lead glass~ means any lead containg
glass, and obviously crystal itself, i.e any ~lass containing at least 24 % lead oxide.
It is well known that any liquid contained in any container or receptacle made of
any material has a tendency to dissolve some quantity of chemical constituents of
20 said material. Said quantity is generally extremely low. As an example, watercontained in a container made of sodocalcic glass will dissolve some Na2O of said
glass. A standard test (DIN 1211) provides a comparison in the hydrolytic resistance
of said sodocalcic glasses (dissolution of 30 to 1000 ,ug Na20 per gram of
powdered sodocalcic glass in water at 90~ C during 1 hour).
25 This general phenomenon is to be considered in analytical chemistry and in
some branches of fine chemistry, but it is generally without incidence in the daily life,
except in very specific situations such as the case of some heavy metals as lead,
baryum and cadmium which have the tendency to drift into the foodstuffs when they
are components of the materials used for making household ustensils or containers
30 to contain beverages. As an example, crystal contains lead oxide (AFNOR
STANDARD: NF 30.004) and it has been ascertained that some amounts of lead,
until some ppm can drift into acid products (pH = 3.5) when said products are kept in
crystal containers during a comparatively long period.
This situation has been seriously investigated and has led to international
35 regulation standards ~in order to ensure the protection of the population against
possible dangers due to the contact between foodstuffs with the surface of articles
made of glass used for the preparation, the service and the conservation of foods
and beverages~ (International Standard ISO 7086/1-1982).
In some countries, the health controlling authorities have now a tendency to
strongly recommend maximal lead contents in alcoholic beverages, which will
become a problem as well for crystal flask producers as for high value alcohol
traders presenting their products in such flasks.
5 A first solution to this problem has been proposéd by the Applicant in the USP4,981,733 and 4,983,199 describing a process for making a container made of
crystal for containing any product and drink with acid characteristics, characterized
in that in order to prevent the lead from migrating from the crystal to the products and
drinks, the container has on its internal surface a thin and continuous film made of
I O unleaded glass forming a ~screen".
Said process comprises the following steps:
- gathering a predetermined quantity of flowable unleaded glass into a drop
shape;
- gathering a parison of crystal over said unleaded glass so as to cover said
I 5 unleaded glass, said leaded glass and said crystal having close the same coefficient
of expansion and index of refraction and,
- blowing said crystal and unleaded glass together to a desired container
configuration having a cavity with an internal surface for holding a product in the
container, the entire internal surface being covered with a continuous film of said
2 0 unleaded glass.
While said solution is quite satisfying, searches have been continued to find out
some alternatives still more attractive.
The present invention is indeed a new process for reaching the same result, i.e to
prevent migration of lead ions contained in a lead glass or crystal container into an
25 acid liquid product contained in said container. This process is based upon a fully
different basic concept, according which instead of isolating the liquid from the
crystal, it is created in a superficial area of flask inner wall, after partially removing
the lead ions contained in said area, a diffusion barrier hindering the remaining lead
ions to pass from the crystal into said acid product.
3 0 To this effect, it is used a well known phenomenon, i.e ion exchange, and more
specifically a cation exchange between lead and alcali metal cations contained in
crystal and H+ and Al3+ cations contained in a material brought into contact with
crystal in specific conditions.
It is indeed known that the hydrolitic attack of glasses by an acid liquid can be
35 considered as an ionic exchange between the H+ in excess in a solution and the
loose cations of the vitreous network, silica remaining practically unchanged. Such
an exchange is practically limited to the network modifiers (alcaline and alcaline-
earth metals) and to some heavy metals such as lead, cadmium and baryum
(inasmuch they are present in the solid).
Starting from this concept, it has already been proposed to limit the migration of
lead by several methods comprising removing the lead from a superficial area of
various thickness in an article wall by using aqueous acid solutions before using
said article. The disadvantage of such methods is to be found in the additionai steps
5 and handlings it gives rise and moreover in the poor results obtained, if in the same
time the safety and time obligations are kept consistent with the industrial production
requirements.
In fact, in the technical field of the lead glasses and crystal compositions the ionic
exchange cinetics between the lead in the wall and the cations in a liquid is
10 governed by the second Law of Fick, i.e the amount of dissolved lead is proportional
to the square root of the contact time, while the diffusion coefficient is an
ARRHENIUS function of the temperature. On the other hand the leaching of articles
by acid solutions can be only carried out at comparatively low temperature, less than
the boiling point of thè used liquids, along practical conditions and contact time
15 limited by the industrial requirements. In addition, the safety requirements make the
use of strongly concèntrated acids practically difficult.
The result thereof is that, in all cases, the amount of lead which is actually
extracted from the container wall is comparatively very small so that a further contact
between the so treated container and an acid liquid results soon again in a diffusion
20 of lead into the liquid which becomes measurable after a few days only: the
diffusion speed remains pratically the same as before treatment, so that said
treatment has for only result a small postponement of the phenomenon, such a
postponement being inadequate in view of the storage periods which can reach
several years.
25 The process of this invention departs from those prior methods and results inpreventing the migration of the lead contained in a lead glass or crystal container
from this container wall into a liquid contained in said container, as well during a
prolonged period, e.g for alcohols contained in crystal flask, as during a
comparatively short period, e.g for strongly acid products such as vinegar containing
3 0 preparations (sauces, mustard) contained in lead glass containers or pots.
To obtain such a result, the process of the invention makes use on the one hand
of high temperatures and on the other hand of compounds capable to exchange or
release H+ and A13+ ions at said high temperature.
Basically, the invention has for its object a process for treating the surface of a
35 solide article made of a material having a vitreous structure, such as leas glasses,
crystal and the same, containing some amounts of heavy metals, such as lead, said
surface being liable to be in contact with acid products, such as alcohols, morespecifically cognac, whisky, wines and fruit juices and similar foodstuffs, with a view
to prevent said heavy metals from drifting or migrating from said surFace into said
$ ~ ~
acid products, said process comprising to create in a superficial area of said solid
liable to be in contact with said acid products, after partially removing the lead ions
contained in said area, a silico-aluminous diffusion barrier opposing said drift or
migration into said product, by ion exchange between said solid article and a layer
5 temporarily applied to said surface, said layer comprising itself a silico-aluminous
crust resulting from heating a kaolin layer applied onto said surface as a thixotropic
barbotin or slip made of pure hydrated aluminium silicate and water.
Practically the process of the invention consists
- in applying upon the surface of the article, liable to be in contact with and acid
I 0 product, a thixotropic slip in the viscous state,
- in emptying the article while keeping on said surface a continuous layer of said
slip adhering to said surface,
- in heating said article with said coating at a temperature between 300~ C and
the softening temperature of the lead glass or crystal forming the article, during a few
15 hours, then, after cooling,
- in c'i."inaling by washing the silico-aluminous crust resulting from said heating.
Barbotin or slip refers to a thixotropic water suspension of compounds such as
phyllosilicates or tectosilioates e.g. clays, kaolins, montmorillonites and zeolites, and
more specifically an hydrated aluminium silicate which retains constitution water up
20 to high temperatures at which the crystal cations are exchanging with the kaolin
cations occurs at a sufficiently high speed to be consistent wiht the industrial time
requirements.
A first explanation of the phenomenon used to carry out the invention would be
the following:
25 The considered cristalline solids are all characterized by structural arrangements
in octaedric or tetraedric layers the overall electric load thereof is deficient in positive
charges due to the replacement of Si4+ cations by less loaded cations such as Al3+.
The excess of negative charges is therefore compensated by other cations, mainlymonovalents (H+, alcaline metals) and sometimes bivalents (alcaline-earth metals).
3 0 Such ~compensating" cations can be exchanged with other species. The number of
negative charges so compensated by exchangeable cations is called "cationic
exchange capacity" (CEC) or Ubase exchange capacity" (BEC).
It should be noted that as from 1945 (cf ~The Glass Industry~, vol. 26 nb 6, June
1945, nb 7 july 1945 and nb 6, June1947), some authors have already shown some
35 possibility of ionic exchange at high temperature between the H~ ions of meta-kaolin
and the alcali metals ions of sodo-calcic glasses, and even considered an
application thereof, i.e an improvement of durability and of some other properties
(mechanical and electrical) of said sodo-calcic glasses.
However, the practical handlings required in such a process have resulted in theabandonment thereof.
Such prior studies have therefore been limited to the exchange between the
sodium of the sodo-calcic glasses and the H+ ions of meta-kaolin, being kept in mind
5 in addition, that the difference between such an ionic exchange at high temperature
and a leaching at room temperature as mentioned above, would be found in the
kinetics of the process on the one hand, and in the surface state after treatment on
the other hand.
Now, it has been shown that with the process of the invention, at the given
l O temperatures between 300~ C and the softening temperature of the lead glass or
crystal forming the article, a few hours contact only is needed to obtain the diffusion
of some amount of the lead of said lead glass or crystal into said silico-aluminous
crust.
In fact, surface analysis by ESCA and SIMS spectrometrics have shown that,
15 during said process it takes place in addition a complex exchange between the K+
and Pb2+ ions of the crystal on the one hand and the Al3+ ions of the kaolin on the
other hand, said exchange, therefore more complex than the one upon which the
first explanation above was based, giving rise in the crystal container wall to a
superficial silico-aluminous layer, of a thickness of about 100 nanometers (10002 0 ~ngsl, oms), forming an interfacial barrier which opposes the inter-diffusion of Pb2+
ions of the solid and H+ ions of the acid liquid in contact. The alumina brought by the
kaolin takes part to the formation of said highly stable superficial layer: such a layer
persists after a thermal treatment at 120~ C during 17 hours, corresponding to the
ageing of several years at room temperature without contact with any liquid.
25 The silico-aluminous crust which remains on the wall at the end of the treatment
is removed, e.g by water washing, completed by some finishing, for instance by sand
blasting, by ultra-sound treatment or the same.
An advantage of this process is the accuracy of its application conditions, which
allow its carrying out upon any article of lead glass or crystal upon industrial3 0 equipment.
The invention will now be described with reference to some practical cases
which demonstrate the efficiency of the process.
Example 1.
This example illustrates a specific application of the process of the invention to
35 the treatment of flasks made of crystal at 30 % lead designed to be filled with
Cognac.
Such flask has a content of 750 ml.
$ &~ ?iL
A barbotin (slip) is prepared from a commercial kaolin sold in France by the firm
Société LAMBERT RIVIERE under trademark NKAOLIN HEAVY". This product is of
high purity, its maximal contents in impurities being:
Calcium.............................. 250 ppm
Heavy metals......................... 25 ppm
Chlorides............................ 250 ppm
SuHates.............................. 0.1 % in weight
Acid soluble su~ nces ............... 10 mg
the other properties being in accordance with the purity required by the
l O European Pharmacopoeia of 1974.
After intimate mixture in a morter of:
Heavy kaolin......................... 47 parts in weight
Water................................ 53 parts in weight
The obtained barbotin (slip) is introduced in the flask and said flask is shaked to
ensure a uniform coating of its entire inner surface. The excess of barbotin is
drained.
The thixotropic properties of the barbotin have for effect that, at rest, the coating
laid on the crystal remains uniform without any later flow.
The flask is then placed in an oven, the temperature of which is progressively
2 0 raised until 400-440~ C and kept at said value during 4-6 hours.
The barbotin crust is then removed by water washing which carries along most
thereof, then with a water/sand suspension to. comp!ete the removing of the lastremaining traces or veil, so that the article recovers its original aspect.
It should be noted that the treatment temperatures are close to the strain point of
the crystal (440~ F), but still slightly lower to prevent any surface alteration or an
excessive adherence of the kaolin crust after coaling.
Generally speacking, the barbotin can contain an amount of water between 40
and 60 % in weight, depending on the specific kaolin used, its granulometry, its way
of application (dipping, brushing and so on), the possible thixotropic additives and
3 0 the shape of the treated article.
In the present specific case, the so treated flask is submitted to a durability test. It
is filled with an aqueous solution containing 4% acetic acid, then the lead content of
said solution is measured by atomic absorbtion every 24 hours.
The same measures are simultaneously made upon an identical flask but having
not been submitted to the treatment.
After 750 hours (one month) the following results are noted:
5~
Flask Pb content of ths solution
Treated non detected
Non-treated 1.70 mg/l
This test is therefore an evidence of the efficiency of the process.
The amount of lead having drifted into the koalin during the treatment is 2-3 mg.
The following tests have for purpose to show that the efficiency of the treatment
5 according to the invention is sufficient for the liquids contained in the treated flasks
remaining in accordance with the sanitary requirements quoted above.
To this effect, in a series of tests carried out along 6, 12, 24, 36 and 60 months
(five years), using as liquids 4 % aqueous acetic acid (pH = 2,30) and a commercial
alcohol such as a standard cognac (pH = 3,5) the following results have been noted,
1 0 as shown on Table 1 and on the curves of the drawing. ,-
TABLE I
A = Acid acetic in 4 % aqueous solution, according to the process of example
1.
B = Alcohol (having an initial own lead content of 16.8 l~g/l)
TEST DURATION
Lead Content of the liquid 6 12 24 36
(~Ig/i) monthsmonths months months5 years
A (Acetic acid tests) 31.043.8 62.0 76 98
B (Alcohol ) 1318.3 25.9 31.6 40
These results (table I and corresponding curves A and B) can be completed by
the following comments:
Firstly, the five year tests are almost superfluous. In fact, the actual age of an
2 0 alcohol such as cognac, as mentioned on a crystal flask does not at all mean that
said alcohol has been kept in said flask all the time during its ageing, which normally
occurs in carboys or casks (generally oak casks), the flask being filled only when
the alcohol is actually offered to the public: the effective contact time between the
alcohol and the crystal is only the display time in the retailer shop, then the
25 customer's consumption time which should logically not exceed some months, at most 2-3 years.
That's why the above tests have been conducted in real time during 12 months,
then in artificial ageing until 36 months and then extrapolated to 5 years, in
accordance wiht the universally recognized Law of Fick, generally admitted in the
3 0 field of ion migration speed, computed in terms of the square root of the time.
Secondly, it should be noted that the lead amounts passed into solution are
substantially less for alcohol than for acetic acid. This is due to the higher pH of
alcohol and the presence of further components in alcoholic beverages such as
cognac.
5 Finally, it is of the utmost importance to oberserve that the highest lead content in
alcohol, after five years, is of 40 ,ug/l only, i.e more than twice lower than the most
severe maximal content edicted up to now.
It can therefore be said that the process of the invention, carried out in the most
extreme conditions, guarantees the most demanding law requirements.
10 It should be well understood that this process is based upon experimental
observations, some of them have been described above and that the present
invention is not limited to and by any theory or hypothetical explanation.
In the present state of our knowledege, it is only possible to remind the different
plausible hypothetic components of an explanation of the efficiency of the process
15 and which are:
1. A simple H+/Pb2+ ion exchange between the crystal and the kaolin. Such a
superficial depletion in lead of the crystal will anyway minimize its later migration
into the liquid.
2. A much more complex exchange between the most important cations of the
20 crystal and the kaolin, generating the formation of a silico-aluminous diffusion
barrier opposing the subsequent migration of the still present lead.
It is highly possible that both components are concomitant, perhaps with furtherones, the first prevailing at the begining of the treatrnent, which means that in a first
stage, in the absence of any barrier, the Pb2+ ions would drift into the kaolin, said
25 superficial depletion in Pb2+ of the crystal being itself a first favourable result, then,
going along the operation progress, initiation and development of said barrier, which
opposes any further subsequent migration of the still remaining lead into the liquid.
