Studies on Copper Red Glaze

Abstract

In the foregoing experiments with regard to the red copper glaze, the influence of the nature of the firing atmosphere and -the time temperature relation upon good coloration was chiefly studied in connection with copper-containing frits whose composition was rather narrowly limited as to the variety of constituents and their proportions. The important facts ascertained through this scientific research may be summarised as follows :-- I) Base composition. Glazes rich in alkalies, and having a comparatively low melting temperature are unsuitable for producing a copper ruby color. On the whole, borosilicate glazes rich in lime and containing 0.25 mols of alumina, and 3.0 mols of silica are considered suitable for the base composition ; thus, 0.25 Na₂O/0.15 K₂O/0.60 CaO } 0.25 Al₂0₂ { 3.0 SiO₂/0.25 B₂0₂ 2) Coloring agent. Within the limits of the experiments, for the glaze composition above cited, 0.3-0.5 % of CuO with an amount of a reducing agent which will at least convert CuO into Cu₂O, produces an excellent red color. The copper content seems to be intimately connected with the percentage of intensifier used if fine results are to be obtained. 3) Intensifiers. From careful experiments it can be concluded that of various color intensifiers ferric and tin oxides give the best results. These two intensifiers, however, seem to give characteristic tones different from each other; iron oxide gives a darker shade, while tin oxide gives a fresh brighter tone. Any agreable red colour may be obtained at will by using both of these together and by changing the proportion of the two. It is considered that a close relation exists between the content. of copper and of the intensifier. So far as the experimental results are concerned, the content of tin oxide should be limited to less than 3 times that of cupric oxide in the case of the glaze above mentioned, otherwise turbidity appears. 4) Firing atmosphere. The nature of the atmosphere should receive close attention. Neither an oxidising nor a reducing atmosphere gives favourable results. Carbon dioxide, nitrogen and steam, which are considered to be neutral at high temperatures, are proved experimentally to be most suitable and a reducing atmosphere is decidedly unsuitable for developing a red color. In ordinary kiln firing, it is perhaps desirable to have complete combustion of fuel with just the theoretical amount of air ; if this is done the hot combustion products containing neither excess oxygen nor reducible constituents, especially when steam is mixed to a certain slight extent therein give fine results. Seger's opinion as regard the best atmosphere, that alternate reducing and oxidising firing is desirable, may be endorsed ; this may produce actually the same effect as the purely neutral atmosphere experimented with by the authors. 5) Color developing temperatures. The cooling rate has also a most important effect upon coloration, as shown by repeated experiments. The analogy of gold ruby coloration may be applicable in this case. There must be the optimum temperature for each glaze composition, and the temperature should be properly maintained in order to give a good color. 6) Proper firing schedule. With glaze of the proper composition, in a neutral atmosphere, a) the firing should be so carried on that the batch composition may fuse to a homogeneous glaze, the temperature being raised to the point where this is accomplished. b) the glaze should next be cooled down to its proper color developing temperature, and c) should be kept at that temperature for a certain time until the color develops to the desired degree, and d) finally it should be naturally cooled.