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Research
Report in support of ASTM D5517

Factors affecting the solubility of metals
in art materials

Woodhall Stopford

Duke University Medical Center

Department of Community & Family Medicine

Division of Occupational & Environmental
Medicine

Durham, NC  27710

John Turner

Angeline Elizabeth Kirby Memorial Health Center

Wilkes-Barre, PA  18701-1391

April 28, 2000

Introduction

Recent studies of mine ore and soils contaminated with lead-containing
mine waste suggest that a major determinant of bioavailable lead is
the amount of soluble lead presented to the small intestine. Lead ore
consists of galena (PbS) altering to anglesite (PbS0<sub>4) or
enclosed in a pyrite or silicate matrix.1 A further study
of the effect of a matrix on solubility has been conducted using a paraffin/stearic
acid wax mixed with two levels of a lead chromate-containing pigment
to form a crayon.

Method ASTM D5517 uses a simple extractant made up of 0.07N HCl. Studies
in this report look at additional factors and components of gastric
and small intestinal juice that might affect absorption including changes
in pH, changes in extractant volume and the effect of sulfhydryl-containing
amino acids, enzymes and organic acids, normally found in gastric and
intestinal juices, on metal solubility. Art materials chosen for these
studies are a lead-containing pigment and a copper-containing ceramic
glaze.

Methods

All reagents used are reagent grade, conforming to the specifications
of the Committee on Analytical Reagents of the American Chemical Society.
A crayon wax containing no pigments and  containing <1 ppm total
lead is used for making lead-containing crayons. A powdered lead chromate
pigment containing 60.4% lead is used for making up test crayons. Crayons
are formulated as follows:

Crayon A  Crayon B

ppm total lead:     640      315

The pigment is mixed with molten wax which is then allowed to solidify
and ground to a particle size of 0.5 mm, sized with a mesh screen.

Weak acid extractions are done in accordance with ASTM D5517.2 
In this method a 100 mg sample is shaken with 5 ml of 0.07 N HCl, the
pH is then adjusted to 1-1.5 with a 2 N solution of HCl and then is
shaken continuously at 37 " 2<sup>o C for 1 hour. After allowing to
sit at 37<sup>o C. for another hour, the mixture is then filtered
through a membrane filter with a pore size of 0.45 m and the eluant tested for lead using flame
atomic absorption spectrophotometry<sup>3  with a detection
limit of 0.02 g/gm of eluant and 5 g/gm of crayon for the entire extraction and
analytical procedure.

To determine the effect of length of weak acid extraction, this extraction
regime is compared with the same protocol except that samples are extracted
for 6 hours with continuous shaking prior to filtration.

Material remaining on filters after the ASTM D5517 extraction is further
extracted with 50 ml of a pH 8.1 buffered aqueous salt solution made
up to approximate that found in the jejunum in non-fasting man<sup>4
with the following composition:

6.79 g NaCl/L

0.52 g NaCl/L

1.85 g NaHCO<sub>3/L

0.11 g CaCl<sub>2/L

1.07 g sodium taurocholate/L

After sonication for 5 min. in an ultrasonic bath, the sample is then
incubated at 37<sup>o C. for 4 h. while being continuously shaken
and then filtered through a membrane filter with a pore size of 0.45 m. The eluant is analyzed for lead as above.

To further judge the effect of amino acids on metal extraction, the
synthetic intestinal juice is amended with sulfhydryl amino acids found
in the small intestine<sup>5,6:

0.718 g glutamate/L

0.271 g histidine/L

The extraction procedure with the amino acid amended synthetic intestinal
juice is the same as for the unamended juice.

In order to determine whether or not the amount of extractant used
has any effect on the amount of metal extracted, the lead chromate pigment
containing 60.4% lead that is used to make lead-doped crayons, above,
and a copper-containing ceramic glaze containing 3.34% total copper
are tested using the standard ASTM D5517 method (1:50 art material to
extractant by weight) and increasing ratios of material to extractant
ranging from 1:200 to 1:315,000. The glaze is formulated with silica-encapsulated
copper designed to prevent tarnishing. To further assess the matrix
effect of wax, the wax in the crayons is removed with trichloroethane
Soxhlet extraction and the remaining pigment analyzed for extractable
lead with ASTM D5517. The amount of extractant used in this procedure
is based on a 1:50 ratio with the crayon, not the remaining pigment.

The extraction results for each sample are calculated in terms of
weight of metal extracted per weight of metal in the art material (percent)
and  ppm of metal that is found in the eluant.

 

Results

A 2 hour weak acid extraction of crayon A resulted in extraction of
5.1-6.1% of available lead in crayon A and 6.0-7.8% in crayon B (Table
1). Little further lead was extracted with a 6 hour extraction or by
using the synthetic intestinal juice extractant, with or without the
addition of sulfhydryl amino acids.

7.9% of available lead in the lead chromate pigment was extracted
using Method ASTM D5517. When the extractant to pigment ratio is increased,
the amount of extracted lead increased until it reached a plateau value
of 87.0-91.5%  of available lead at a pigment to extractant ratio
of 1:100,000 or greater. The amount of soluble lead in the eluant is
5.3 ppm or less once this plateau is reached (Table 2). When the wax
is removed from the crayons, the soluble lead level in the remaining
pigment is 84.3% for the pigment from crayon A and 90.5% for the pigment
from crayon B (Table 1). The concentration of lead in the eluant for
these two extractions was 10.8 ppm for pigment from crayon A and 5.7
ppm for pigment from crayon B.

With the copper-containing glaze, extractable copper levels increased
as well plateauing  at a value of 12.6-13.5% of available copper
once the glaze to extractant ratio reached  a value 1:1000. At
this level the eluant contained 4.2 ppm copper (Table 3).

Discussion

A number of experiments were conducted with lead chromate-containing
crayons to see if changing the extraction procedure to test whether
or not elements that might better represent what occurs in the stomach
and intestine would appreciably increase extraction amounts. Increasing
the extraction times by a factor of 3 resulted in some increase in the
percent extractable lead as did further extraction with synthetic intestinal
juice, with or without the addition of sulfhydryl amino acids. The amount
of increased extractable lead, however, was near or below the level
of detection for the method.

When looking at
the lead chromate pigment used in these wax crayons, the amount of extractable
lead increased with increasing extractant to pigment r