GLAZES

 

 

Here a colouring experiment made on a crystalline glaze with Zinc-Barium.       Chemical formula of the glaze :   Oxide Molar ratio / Seger CaO 0,065 ZnO 0,575 BaO 0,091 Na2O 0,244 K2O 0,025 Al2O3 0,085 SiO2 1,805 TiO2 0,160

additives and colouring agents : V2O5 1,0 % - Sb2O3 1,5 %

On the photograph above one sees perfectly well that the crystalline glaze simultaneously developed two distinct crystalline phases on a pinkish opaque background (opacified by titanium). The additives and dyes used are the vanadium pentoxide (V2O5) and the antimony trioxyde (Sb2O3). a click on the picture to enlarge it A phase consists of brown-pinkish coloured crystals (the largest one) with a blackish fringe, the other consists of subjacent white silvery crystals with a violet fringe.   Two crystalline phases thus develop simultaneously during the growth stage.   See this glaze on an egg in full screen

on this picture, the same glaze was coloured with 1% of vanadium pentoxide (V2O5) and 1,5% of bismuth trioxyde (Bi2O3). Only one type of crystal developed, straw yellow color on a cream opaque background (opacified by titanium).       a click on the picture to enlarge it

additives and colouring agents : V2O5 1,0 % - Bi2O3 1,5%

Firing schedule - Peak at 1255°C

THE KILN   This small electric kiln has a 230 volts single-phase current supply, with a power of 3400 Watts and a capacity of 30 liters. The insulating walls are made of refractory ceramic fibers for high temperature (Cerachem blanket - 1450°C - by Thermal Ceramics) with a 220 mm thickness. The roved resistances assembled on tubes are made starting from an Fe-Cr-Al alloy in 2 mm diameter wire. A regulator-programmer Omron E5CK-T controls the kiln starting from a platinum sensor (type S). The power is distributed by a static relay Crydom. The firing curve used for the formation and the growth of the crystals is the same one for the two coloured glazes. They were fired simultaneously in the kiln.

THE GLAZE   Several properties could be calculated starting from the chemical formula of the glaze: - the eutectic (passage in vitreous phase) is given for 1106°C. This value is appropriate completely for a peak of temperature fixed at 1255°C. - the linear thermal expansion coefficient: 76 X 10 -7 °C-1 - the viscosity of the glaze at the various temperatures of the firing curve::   Temperature Viscosity in Poises 1255°C 158 Po 1080°C 1106 Po 1050°C 1628 Po 1020°C 2442 Po 800°C 97338 Po !!!

THE FRINGE OF CRYSTALS   The fringe of crystals is formed in the last part of the growth stage during the 2 hours at 1020°C. When passing at this temperature, the viscosity of the glaze strongly increases (it passes from 1106 Po to 2442 Po). The mobility of the elements is slowed down and the construction of the crystal is strongly modified in form and color. After these two hours, the heat is cut and the kiln cools quickly (between 150 with 200°C per hour) :   Blackish fringe    Violet fringe   Gilded fringe

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