Preparation of artificial turquoise pellets by hydrothermal hot pressing method

Onoda, Hiroaki; Nishimura, Ayaka

doi:10.1590/PPJA9080

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ORIGINAL ARTICLE • Cerâmica 70 • 2024 • https://doi.org/10.1590/PPJA9080 copy

Preparation of artificial turquoise pellets by hydrothermal hot pressing method

Authorship SCIMAGO INSTITUTIONS RANKINGS

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

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

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

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

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

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

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

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[1] * h-onoda@kpu.ac.jp