Preparation of artificial turquoise pellets by hydrothermal hot pressing method

Abstract

Phosphate materials based on Turquoise composition CuAl₆(PO₄)₄(OH)₈•4H₂O were prepared using aqueous solutions. The light blue or green powders were obtained by heating at 500 or 700 ºC. To obtain artificial Turquoise pellets, these were sintered by the hydrothermal hot press method at 150 ºC. The amount of water, pressure, and milling as pretreatment were studied. Molded pellets were also prepared using oil as the solvent. The resulting pellets were evaluated in terms of density and hue. In addition, the effects of post-treatment, such as heating and beeswax coating, were also examined. There were several methods to improve the density of the molded product, including increasing the amount of solvent, increasing the pressure, using castor oil, and powder refining as a pretreatment. Castor oil and beeswax coating were also methods to improve product brightness. The pellets obtained in this study showed water resistance.

Keywords:
turquoise; artificial ornament; hydrothermal hot pressing

INTRODUCTION

Some natural ores are used as ornaments because of their beautiful luster and color ^{1), (2}. Although transparent natural ores such as diamonds are well known as ornaments, opaque ones are also used as ornaments ^{3), (4}. Turquoise is a blue to green natural ore, and the best ones are used as ornaments. Blue are generally considered more valuable, but green ones are more valuable in some regions. There is much demand for turquoise, but the supply of natural, high-quality products is limited. For this reason, artificial turquoise has been produced and supplied to the commercial market ^{5), (6}. However, problems have arisen, such as low quality products that are simply coated with other ores and sold as turquoise. These are inferior products, and there is a demand for better products, even as inexpensive imitations.

Natural ores have existed through exposure to wind and rain and have a certain degree or more of chemical stability. The authors have been developing new pigments without toxic heavy metals and have prepared inorganic phosphate pigments that mimic the composition of natural ores ^{7), (8}. In this context, we have successfully prepared blue and green pigments that mimic the composition of Turquoise (CuAl₆(PO₄)₄(OH)₈•4H₂O) by precipitation in aqueous solution ⁹. Since this study involves the preparation of shaped artificial ores rather than powdered pigments, the method of preparing the compacts was considered. The most common sintering method, pressure-less sintering, requires high temperatures. On the other hand, phosphate volatilizes phosphorus oxide when heated to high temperatures, transforming it into an oxide ^{10), (11}. Thus, pressure-less sintering is not suitable for sintering phosphates. We have previously prepared phosphate compacts by a hydrothermal hot pressing method to evaluate their conductivity ^{12), (13}. This method obtains compacts at relatively low temperatures by mixing a small amount of water with the sample powder and applying pressure ^{14), (15}. Thus, phosphate can be processed into molded products without decomposition.

In this study, the phosphate pigment was molded to create a high-quality artificial turquoise. Since turquoise composition powder turns green when heated, the light blue and green moldings were produced by varying the heating temperature as a pretreatment. It is expected to supply the market with high-quality artificial turquoise by applying a surface coating to the obtained artificial turquoise.

EXPERIMENTAL

Target material imitated of Turquoise, CuAl₆(PO₄)₄(OH)₈•4H₂O, was prepared in the following reactions.

$$\begin{gathered} \mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2+6\mathrm{Al}{\left({\mathrm{NO}}_3\right)}_3+4{\mathrm{H}}_3{\mathrm{PO}}_4+8\mathrm{NaOH}\to \\ {\mathrm{CuAl}}_6{\left({\mathrm{PO}}_4\right)}_4{\left(\mathrm{OH}\right)}_8•4{\mathrm{H}}_2\mathrm{O}+8{\mathrm{NaNO}}_3+12{\mathrm{HNO}}_3 \end{gathered}$$ (A)

A total of 0.5 mol/L of copper and aluminum solutions were prepared from copper nitrate and aluminum nitrate in the ratio of Cu/Al=1/6. Then, 0.5 mol/L of phosphoric acid was added to this copper and aluminum mixed solution in the ratio of Cu/Al/P=1/6/4 (Turquoise ratio). The mixed solutions were adjusted to pH 5 with 8 mol/L of sodium hydroxide solution. The precipitates were filtered and then dried. The precipitates were heated at 500 or 700 ºC for 1 hour under air conditions. All chemicals were of commercial purity (FUJIFILM Wako Pure Chemical Corp., Osaka, Japan) and were used without further purification. A part of the phosphate powder was treated with a ball mill (BALL MILL ANZ-10S, Nitto Chemical Co. Ltd., Tokyo, Japan) at 300 rpm for 1 hour. The polypropylene pot (47 mL) and 6 balls (alumina, 10 mm, average 2.1 g) were used for milling 4 g of sample powders.

The mixtures of heated samples (1.0 g) and water (0-0.7 ml) were placed in a mold (inner diameter: 14 mm) and mechanically pressed with a uniaxial pressure of 15 - 35 MPa at 150 ºC for 1 hour (Hydrothermal hot pressing process, HHP process). Because the mold had a slight excess of space, water could be volatilized in the sintering process. Since some natural ores contain mineral oil, castor oil was also used in the production of the moldings instead of water. The size of the obtained molded body was measured using a caliper, and the density was calculated from its volume and weight.

To evaluate the water resistance of the sample pellets, the petri dish was filled with water and the pellets were placed gently in it. The pellets did not decay even after being placed quietly for 5 minutes, confirming that they were sufficiently water-resistant. Since ornaments are not usually soaked in water for a long time, this time was judged to be sufficient for water resistance evaluation.

The chemical composition of these materials was analyzed using X-ray diffraction (XRD) and infrared (IR) spectra. XRD patterns were recorded on an X-ray diffractometer (MiniFlex, Rigaku Corp., Akishima, Japan) using monochromatic CuKα radiation (30 kV, 15 mA, 3º/min). The IR spectra of the samples were recorded by the KBr disk method (resolution: 4 cm^-1, 16 times scanned) using a HORIBA FT-IR 720 (HORIBA Corp., Kyoto, Japan). The particle size distributions of these materials were measured using a centrifugal precipitation particle-size distribution (SA-CP3L, Shimadzu Corp., Kyoto, Japan). The hue of pellets was estimated in L*a*b* color space with TES135 plus color analyzer (TES Electrical Electronic Corp., Taipei, Taiwan) (average of 5 times). The L* value refers to the whiteness of the powder, with 100 being white and the opposite being 0 being black. The a* value signifies the redness of the material, with positive and negative values corresponding to red and green, respectively ¹⁶. The b* value denotes yellow intensity, in which positive and negative values correspond to yellow and blue, respectively.

RESULTS AND DISCUSSION

Powder materials: Figure 1 shows XRD patterns of samples prepared at various temperatures. Samples without heating had the peaks of Turquoise and CuO ¹⁷. On the other hand, samples heated at 500 and 700 ºC were amorphous. Figure 2 shows the IR spectra of samples heated at various temperatures. The IR spectrum of the unheated sample has a peak at 1650 cm^-1, which is due to water. This peak disappeared due to the volatilization of water in the sample by heating. The sharp peaks at 850 and 1390 cm^-1 were due to nitrate ions and disappeared by heating at 700 ºC ¹⁸. Samples contained nitrate ions because they were prepared using aqueous solutions containing nitrate ions. The broad peak around 1080 cm^-1 was due to phosphate ions, indicating that the sample contained phosphate ¹⁷. Figure 3 shows the photograph of samples heated at various temperatures. The sample without heating had a pale light blue color, which became darker with heating at 500º C. The sample became green when heated at 700º C.

Figure 1:
XRD patterns of samples heated at various temperatures, (a) without heating, (b) 500 ºC, (c) 700 ºC, ▼; Turquoise, ○; CuO.

Figure. 2:
IR spectra of samples heated at various temperatures, (a) without heating, (b) 500 ºC, (c) 700 ºC.

Figure 3
Photographs of samples heated at various temperatures, (a) without heating, (b) 500 ºC, (c) 700 ºC.

Pressing: Figure 4 shows the photographs of hydrothermally hot pressed samples. Samples heated at 500 ºC and 700 ºC formed light blue and green bulks, respectively. Table 1 shows the density and L*a*b* values of phosphate bulks prepared with various volumes of water. The increasing amount of water increased the density of the pellets. It was thought that the increased amount of water would allow the particles to move more easily, forming pellets with fewer gaps. The sample heated at 500 ºC was able to form pellets even if a relatively large amount of water was added. For the samples heated at 500 ºC, the addition of water during molding increased the L* value, ie the color of the sample pellets became lighter. The a* and b* values decreased, indicating a greener and bluish tint, respectively. The formation of denser pellets corresponded to a decrease in the gaps within the pellets, which was thought to make the color appear more clearly. For the sample heated to 700 ºC, a molded body could be obtained only up to the condition of 0.2 mL of water, and no regularity in color change was observed.

Figure 4:
Photographs of typical samples pressed from powders heated at various temperatures, (a) 500 ºC, (b) 700 ºC.

Table 1
Density and L*a*b* values of phosphate bulks prepared with various volumes of water (150 ºC, 20 MPa, 1 h)

Pre-heating	Water volume mL	Density gcm-3	L*	a*	b*
500 ºC	0	1.26	68.3	-8.5	-10.6
500 ºC	0.1	1.35	74.6	-15.5	-14.3
500 ºC	0.2	1.37	76.8	-16.5	-16.0
500 ºC	0.3	1.38	82.3	-18.4	-16.4
500 ºC	0.4	1.40	78.9	-15.6	-15.5
500 ºC	0.5	1.59	74.4	-23.7	-24.9
500 ºC	0.6	1.63	76.5	-19.8	-21.8
500 ºC	0.7	1.37	-	-	-
700 ºC	0	1.52	73.5	-14.5	7.6
700 ºC	0.1	1.53	74.0	-15.2	7.4
700 ºC	0.2	1.76	61.6	-9.5	7.8

Table 2 shows the density and L*a*b* values of phosphate bulks prepared under various pressures. The amount of water was set to the condition where the density is the highest in Table 1. By increasing the pressure, the density of the pellet was improved. However, it was lower than the actual density of Turquoise (2.6-2.9 g/cm3). In the hydrothermal hot pressing method, it is known that the added water volatilizes, resulting in a somewhat low-density sintered body ¹⁵. Among samples heated at 500 ºC, the pellet molded at 35 MPa was the most greenish and bluish. The sample heated at 700 ºC showed relatively little change in color due to pressure.

Table 2
Density and L*a*b* values of phosphate bulks prepared under various pressures (150 ºC, 1 h)

Pre-heating	Water volume mL	Pressure MPa	Density gcm-3	L*	a*	b*
500 ºC	0.6	15	1.62	77.3	-18.7	-19.3
500 ºC	0.6	20	1.63	76.8	-16.5	-16.0
500 ºC	0.6	25	1.65	71.5	-24.1	-25.7
500 ºC	0.6	30	1.61	70.5	-18.6	-15.9
500 ºC	0.6	35	2.01	67.2	-26.0	-30.8
700 ºC	0.2	15	1.71	56.2	-11.1	8.3
700 ºC	0.2	20	1.76	61.6	-9.5	7.8
700 ºC	0.2	25	1.82	59.6	-11.5	7.5
700 ºC	0.2	30	1.84	70.4	-11.3	6.8
700 ºC	0.2	35	1.90	61.2	-8.9	7.2

Since natural ore maintains its shape with mineral oil, the compact was produced using oil instead of water. Table 3 shows the density and L*a*b* values of phosphate bulks prepared with various solvents. The density of compacts was improved by using castor oil. The L* and a* values decreased, while b* values increased. The L* values closer to 100 are closer to white, and closer to 0 are closer to black. Therefore, further away from either value means a brighter color. In this study, the color became vivid by using castor oil. A decrease in a* value meant an increase in greenness, and an increase in b* value meant a decrease in bluishness. This color change may be related to the light yellow color of castor oil itself.

Table 3
Density and L*a*b* values of phosphate bulks prepared with various solvents (Heating; 500 ºC, Pressing; 150 ºC, 1 h, 20 MPa)

Solvent	Volume mL	Density gcm-3	L*	a*	b*
water	0.2	1.37	76.8	-16.5	-16.0
water	0.3	1.38	82.3	-18.4	-16.4
castor oil	0.2	1.59	74.6	-22.2	-11.4
castor oil	0.3	1.61	76.8	-23.0	-12.9

Pretreatment: to obtain higher densities, powder refinement was carried out by ball milling before molding ¹⁹. Figure 5 shows the particle size distribution of samples heated at 500 ºC and then treated with a ball mill. The milling slightly decreased the percentage of larger particles than 10 μm and slightly increased the percentage of smaller particles than 4 μm. Samples heated at 700 ºC showed little change due to milling. It was considered that this was because the particles were hardened by being heated to a high temperature.

Figure 5
Particle size distribution of samples heated at 500 ºC and then treated with ball-mill, (a) 0 h, (b) 1 h, (c) 6 h.

Table 4 shows the density and L*a*b* values of phosphate bulks prepared with various milling hours. Samples milled for 6 hours formed denser compacts than unmilled samples. The L* values were almost unchanged by milling. Samples heated at 500 ºC showed a significant decrease in a* and b* values by milling and increased greenish and bluish colors.

Table 4
Density and L*a*b* values of phosphate bulks prepared with various milling hours (150 ºC, 1 h)

Pre-heating	Milling h	Density gcm-3	L*	a*	b*
500 ºC	0	1.63	76.8	-16.5	-16.0
500 ºC	1	1.56	74.8	-24.4	-26.1
500 ºC	6	1.76	74.2	-24.4	-26.0
700 ºC	0	1.76	61.6	-9.5	7.8
700 ºC	1	1.70	61.6	-6.5	9.6
700 ºC	6	1.84	61.0	-5.8	9.3

After treatment: sintering was attempted as a post-processing step after hydrothermal hot pressing to obtain denser pellets ²⁰. Table 5 shows the density and L*a*b* values of phosphate bulks before and after sintering at 500 ºC for 1 hour. No densification due to sintering was observed.

Table 5
Density and L*a*b* values of phosphate bulks (150 ºC, 1 h, 20 MPa) before and after sintering at 500 ºC for 1 hour /g•cm-3

Pre-heating	solvent	Volume of solvent mL	Density before sintering gcm-3	Density after sintering gcm-3
500 ºC	water	0.5	1.59	1.49
700 ºC	water	0.2	1.65	1.67
500 ºC	oil	0.2	1.59	1.57

To add luster and avoid hydrolyses, the pellets were dipped and soaked in melted beeswax ²¹. Table 6 shows L*a*b* values of phosphate bulks before and after coating with beeswax. Pellets prepared with water had an L* value of about 50 and a distinct color due to beeswax coating. In particular, the pellets of a sample heated at 500 ºC had lower a* and b* values, i.e., more greenish and bluish. Density is higher due to beeswax seepage; however, this was not measured as a comparison with turquoise is not meaningful.

Table 6
L*a*b* values of phosphate bulks before and after coating with beeswax

Pre-heating	solvent	coating	L*	a*	b*
500 ºC	water	before	76.8	-16.5	-16.0
		after	56.0	-19.7	-22.9
500 ºC	oil	before	75.6	-22.2	-11.4
		after	34.3	-15.3	-7.0
700 ºC	water	before	61.6	-9.5	7.8
		after	50.4	-12.4	7.1

CONCLUSIONS

The hydrothermal hot pressing method was used to obtain a molded product that mimics the Turquoise composition. The methods to improve the density of the compacts included increasing the amount of solvent, increasing the pressure, using castor oil, and powder refinement as a pre-treatment. In addition, castor oil and beeswax coating are methods to improve the vividness of the product. The pellets obtained in this study exhibited water resistance.

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Edited by

(AE: D. Z. de Florio)

Publication Dates

• Publication in this collection
  12 Aug 2024
• Date of issue
  2024

History

• Received
  25 Sept 2023
• Reviewed
  03 Nov 2023
• Accepted
  09 Nov 2023