Final Report: Elimination of Lead from Ceramic Glazes by Refractive Index Tailoring

EPA Grant Number: R830420C008
Subproject: this is subproject number 008 , established and managed by the Center Director under grant R830420
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Center for Environmental and Energy Research (CEER)
Center Director: Earl, David A.
Title: Elimination of Lead from Ceramic Glazes by Refractive Index Tailoring
Investigators: Carty, William
Institution: Alfred University
EPA Project Officer: Lasat, Mitch
Project Period: May 1, 2006 through April 30, 2007
Project Amount: Refer to main center abstract for funding details.
RFA: Targeted Research Center (2002) Recipients Lists
Research Category: Congressionally Mandated Center , Targeted Research

Objective:

The proposed concentration gradient giving leaded glazes an added optical brilliance that has not been found in unleaded glazes was the theory set to be tested.  The use of a dual glazing application process will be tested with the goal of chemically tailoring the composition to create a barium concentration gradient increasing towards the free surface.  This composition gradient was expected to yield a similar optical appearance to that observed in Pb-containing glazes.  Four series of experiments were performed in an attempt to obtain the compositional gradient. Compositional gradients were obtainable, yet the mechanisms of diffusion prevented the control of distant thickness of the layers within the gradient.  Crazing of the glazes became an issue in several of the experiments.  The optimal glaze was not obtained.  

Background

The toxicity of lead has been known for over 2000 years.  Lead affects both humans and the environment.  Since lead does not decay or biodegrade this results in a build up of a long term pollution source.  PbO is a strong fluxing oxide that is still found in commercial ceramic dinnerware glazes today.  In April 1992, lead in ceramic glazes was listed as a concern in a hazard summary.  Advances in characterization is creating a trend for tighter lead leaching standards.  It is well documented that leaded glazes have advantageous properties of greater brilliance, chemical durability and smooth surface.  In the dinnerware industry the visual appearance of a glaze is of such extreme importance that many dinnerware manufacturers are reluctant to change to unleaded glazes.

The high brilliance in leaded glazes is usually explained by an increase of index of refraction.  Index of refraction is a function of electron density and polarizability.  The electron density is directly related to atomic weight and polarizability is related to ion size.  Pb2+ ion has a high atomic number and large ion size that accounts for the high index of refraction found in leaded glazes.  CaO is the principle alkaline earth fluxing oxide used in non-leaded glaze due to its cost and availability.6 The substitution of other fluxing oxides of BaO or SrO for CaO have been historically recommended.

Mejia was able to obtain high gloss values with the substitution of SrO and BaO.  Gloss units are measured with a glossmeter.  Gloss units is a function of the amount of reflected light from a black glass standard with a defined refractive index (1.567).  Quinlan was able to relate the gloss measurements to the rms roughness values obtained through interferometry as illustrated in Figure 1.  It should be noted that at high gloss values a significant scatter still exists in the correlation to surface roughness.  Mejia also cited limitations of the different spectroscopy methods and discrepancy in quantitative values.  Even though no measurable quantitative difference can be placed between a leaded and non-leaded glaze, the visual appearance of a lead-free glaze is still accepted to be inferior to a leaded glaze.

Relationship of glossmeter measurements to  interferometer measurements.

Figure 1. Relationship of glossmeter measurements to interferometer measurements.

Research by Woods showed the possibility of other factors contributing to the advantageous properties found in leaded glazes.  Woods found the lead concentration of an industrial glaze was greater at the glaze surface, as shown in Figure 2.  A higher lead concentration at the surface of the glaze would indicate an index of refraction gradient.  Using glass property calculation software (SciGlassä v 3.5, SciVision Inc, Burlington, MA) the Winkelman and Schott method was applied to calculate the index of refraction of the compositions found by Woods and was plotted as a function of position within the glaze (Figure 3).  The existence of a refractive index gradient is proposed to explain the superior optical quality of leaded glazes.  This higher lead concentration would also lead to a lower viscosity at the glaze surface.  Lower surface viscosity will also result in improvements in the self-leveling characteristics producing a smoother surface.  These two factors, refractive index and smoothness, combine to produce a superior surface quality that would be visible to an observer, but difficult to quantify experimentally.  With an understanding of how lead glazes differ from lead-free glazes, the characteristics of a leaded glaze can be imitated.

Lead concentration in the cross section of a  body and glaze by electron dispersive spectroscopy.

Figure 2.  Lead concentration in the cross section of a body and glaze by electron dispersive spectroscopy.

Calculated refractive indices within the  glaze.

Figure 3.  Calculated refractive indices within the glaze.

As a part of this study a thorough sampling of industrial glaze samples was performed.  In all cases the melt behavior, microstructure (bubble size and distribution, crystallization events) within various locations (top and bottom), was found to be uniform, thus confirming the concentration gradient hypothesis was not based on an anomalous event.

The lead concentration gradient found in industrial glazes, proposed by Woods from Electron Dispersive Spectroscopy (EDS) data correlated with the Wavelength Dispersive Spectroscopy (WDS) is illustrated in Figure 4.  The WDS Elemental Map provides a better method to visualize and quantify the chemical concentration gradient in comparison to other techniques such as EDS spot analysis or EDS line scans.  EDS is also a semi-quantitative technique accurate to 0.1 wt% in its optimal scan conditions.  WDS, when used with the custom standards, can be quantitatively accurate to the 100 ppm (0.0001 wt%).  Quantitative standards of intermediate concentrations are being prepared.

Figure 4. Wavelength Dispersive Spectroscopy Elemental Map for lead.  The body-glaze interface is the blue line on the right.  The free glaze surface is on the left.  The Pb concentration increases towards the free glaze surface as evidenced by the shift in color towards red.  Color denotes relative concentration: red ≡ high; blue ≡ low; black < detection limit.

Methods:

The first two series of experiments involved the use of dual glazing process using two different glaze compositions to generate the gradients based on melt behaviors.  The first glaze layer is to facilitate body-to-glaze interaction and prevent bubble formation.  The application of the second layer will be a higher melting temperature glaze to limit diffusion of BaO or SrO from the top layer to the bottom layer.  By limiting the diffusion, a chemical and refractive index gradient will be introduced.  BaO and SrO are aggressive fluxes at higher temperatures, and SrO is known to increase the fluidity of a glaze when substituted for CaO.  The use of delayed melting will imitate the self-healing and segregation nature of leaded glaze.

Two different interfacial glaze compositions, and seven different top layer glazes of varying barium concentrations have been tested and are shown in Table I.  Series A glazes were composed of a thin interfacial layer (~50 mm) and thicker barium top layer (~150 mm); Series B glazes were composed of a thick interfacial layer (~150 mm) with a thin barium top layer (~50 mm).  The control of the glaze layers was performed by measuring the mass glaze layers in green to predict the applied thickness.  Glazes were applied to fully dense Niagara Ceramic tiles and fired to 1270°C at 3°C per minute for a 2-hour dwell.  Samples were measured for gloss values by a gloss meter and cross-sectioned and polished for Wave Dispersive Spectroscopy.  An Electron Probe Microanalyzer with a wavelength-dispersive X-ray spectrometer (WDS) was used for the analysis.  Calibration standards containing varying concentrations of barium were made and submitted for chemical characterization (ACME Analytical Laboratories, Ltd., Vancouver, BC).

Table I. Compositions of Layers in UMF for the First Two Studies

 

Interfacial Layer

Top Layer

 

I1

I2

T1

T2

T3

T4

T5

T6

T7

SiO2

2.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

Al2O3

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

R2O

0.6

0.6

0.15

0.15

0.15

0.15

0.15

0.15

0.15

CaO

0.4

0.4

0.7

0.6

0.5

0.4

0.3

0.2

0.1

BaO

0

0

0.15

0.25

0.35

0.45

0.55

0.65

0.75

The third series of experiments tested a theory of  “charge coupling effect” to slow the barium diffusion.  Small additions of B2O3 were made to lower the melting temperature of the glazes, since the glaze maturity of the previous trials seemed to have not been reached.  The dwell was also cut to one hour since diffusion is a time-dependent phenomenon.  Data from the previous trials showed that BaO concentration of 0.25 UMF is sufficient; also, increasing BaO concentrations will make charge coupling more difficult to achieve.  The glazing process from Series B was applied.   The compositions for the third study are shown in Table II.  Glazes were then evaluated with the use of a gloss meter and samples were then cross sectioned and polished for WDS.

Table II. Compositions of Layers in UMF for the Third Study .

 

Interfacial Layer

Top Layer

 

I3B

T1

T2

T3

T4

T5

T6

T7

SiO2

2.35

2.9

3.07

3.24

3.41

3.58

3.75

3.92

Al2O3

0.35

0.5

0.5

0.5

0.5

0.5

0.5

0.5

B2O3

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

R2O

0.3

0.15

0.15

0.15

0.15

0.15

0.15

0.15

CaO

0.7

0.6

0.6

0.6

0.6

0.6

0.6

0.6

BaO

0

0.25

0.25

0.25

0.25

0.25

0.25

0.25

Results:

Gloss values for series I1, I2A, I2B are reported in Table III.  The I1 interfacial layer approached a maximum gloss value as the barium concentration increased in the top glaze layer from T1 to T4.  Increasing the barium concentration past 0.45 gloss values saw a large drop, further illustrating the maximum and a concentration at T4.  No values were obtained for the T7 glaze the on the I1 interfacial layer because the glaze crawled.  The I2A interfacial layer also experienced a maximum gloss which occurred at a higher barium concentration, relative to I1, being approximately 0.55.  The maximum value for the I2B interfacial glaze layer is approximately 0.55, same as the I2A layer.  The gloss value for the T4 top glaze on the I2B interfacial layer is an outlier and was not taken into consideration.  Increasing barium concentration past 0.65 BaO UMF for the top glaze (T6) was too high for a desirable effect for any interfacial layer.

Table III. Gloss Unit Values for I1, I2A, I2B Series.

 

I1

I2A

I2B

T1

92.4

86

79.4

T2

93.3

85.1

92.1

T3

90.1

89

88.8

T4

92.5

92.1

77.3

T5

83.6

91.9

90.1

T6

81.6

91.9

89.8

T7

N/A

75.6

83.9

*Samples in bold print underwent WDS analysis.

WDS analysis for the I1-T1 glaze combination showed a large concentration gradient of calcium throughout the glaze layer.  High concentrations of calcium are seen at the top layer while low concentrations are seen at the glaze-body interfacial layer.  Smaller concentration gradients also occurred through the glaze layer using this combination; seen in aluminum, sodium, potassium, and barium.

WDS analysis for the I1-T3 combination also showed a large concentration gradient for calcium, again with the higher calcium concentration at the glaze surface and the lower concentration at the interfacial layer.  The concentration gradients for aluminum, sodium, potassium, and barium are more pronounced in this combination relative to the I1-T1 combination.

WDS analysis for the I1-T6 glaze combination shows less of a concentration gradient for every element studied in comparison to sample I1-T3 and I1-T6.  The high barium concentration in the T6 glaze is most likely accountable for the effect seen; the barium most likely decreased the viscosity of the entire glaze layer allowing for diffusion to occur more rapidly.  The areas of high barium concentrations were found in the body rather than the glaze layer.

WDS analysis for the I2B-T1 glaze combination shows a well defined concentration gradient for calcium throughout the glaze layer.  Defined, however slight, concentration gradients for aluminum, sodium, potassium, and barium are also seen throughout the glaze layer.

WDS analysis for the I2B-T3 glaze combination shows a larger concentration gradient for calcium throughout the glaze layer in comparison to the sample I2B-T1 of lesser barium concentration.  High concentrations of calcium are found at the surface while the area of low calcium concentration is found at the glaze-body interface.  Very slight concentration gradients exist for aluminum and barium.

WDS analysis for the I2B-T6 glaze combination again showed a distinct calcium concentration gradient where the interface glaze meets the top glaze similar to that found in I2B-T3.  All other elements studied show no concentration gradients with the glaze layer.

The reported gloss values for series I3B are reported in Table IV.  Gloss units above 95 were reported for the series.  Sample I3BT4 was prepared for WDS analysis.

Table IV. Gloss Unit Values for I3B.

Sample

Gloss Units

I3BT1

96.4

I3BT2

96.2

I3BT3

98.6

I3BT4

98.7

I3BT5

97.3

I3BT6

96.0

I3BT7

95.9

WDS analysis for the I3-BT4 glaze combination shows no concentration gradients for any element within the glassy glaze layer.  Crystallization started to occur near the glaze-body interface which can be seen in the micrograph by a variance in the concentration of barium.  The high gloss readings in this series were not caused by  the existence of a chemical gradient.

Conclusions:

This study illustrated the ability to tailor concentration gradients throughout the glaze layer on a ceramic body.  The optical effect observed in leaded glazes was not achieved in this study.  However, this study does prove that the effect is achievable without the use lead by tailoring the concentration gradients.

Supplemental Keywords:

unleaded glazes, index of refraction, leaded glazes, gloss values, lead-free glaze, refractive index gradient, WDS elemental map, chemical concentration gradient,, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Sustainable Industry/Business, Sustainable Environment, Environmental Chemistry, Technology for Sustainable Environment, pollution prevention, cleaner production, environmental sustainability, alternative materials, lead free ceramic glaze, refractive index tailoring, environmentally benign alternative, environemntally benign lead substitute

Relevant Websites:

http://ceer.alfred.edu Exit


Main Center Abstract and Reports:

R830420    Center for Environmental and Energy Research (CEER)

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