Surface:
The enamel coating shall have a uniform, smooth, fully fused surface free from impurities.
Vitreous enamel thickness:
The thickness of enamel coatings on steel substrates is to be kept within the range of 1.0 mm to 2.2 mm, with the following exceptions:
1) On concave surfaces, the maximum values may be exceeded by 0.2 mm;
2) On convex surfaces, the enamel coating may be 0.2 mm thinner than the specified minimum value, but only in limited areas, not overall;
3) Coatings on small parts with very small radii < 5 mm, such as valve stems or pump rotors, may have aminimum thickness of 0.6 mm. The differences of enamel thickness on one piece are to follow a smooth transition. The total enamel thickness is reached by one or two layers of ground coat (approx. 0.15 mm for each layer) and multiple cover coat layers (approx. 0.2 mm) until the minimum thickness is reached.
Unacceptable defects:
If the cover coat shows one or more of the following defects, another layer must be applied or enamelling must be restarted:
1) collapsed lines in the cover coat;
2) bubbles, bubble lines, i.e. fused-in bubbles arranged in a distinct line;
3) fused strain lines (recognizable as lines with colours different from that of the surrounding enamel);
4) areas not properly fused (a rough surface similar to a sandblasted one);
5) devitrified areas invitreous enamel or over crystallized areas in semi-crystallized enamel (recognizable as a dullor a rough surface);
6) pull-through of ground coat(recognizable as e.g. spot-like discolorations);
7) detectable cracks;
8) spots with a diameter >30 mm caused by grinding and polishing during the removal of impurities.
Firing of the vitreous enamel:
Firing is to be started only when the applied coat is completely dry. To avoid sudden evaporation of water, the furnace temperature should be lower than 100℃ when the chemical apparatus is moved into the furnace. We recommend to have a holding time of 15 to 20 min. at about 575℃ to allow quartz to transform its structure. A good rate for heating up is about 10℃/min. The ground coat can be fired up to 900℃. The firing temperature of each enamel layer must be 10℃ lower than the previous layer,otherwise the enamel may slip / move away on the vessel.
The maximum firing temperature for the cover coat is 840℃. The final layer has a minimum firing temperature of 790℃. After firing, the vessel should not be cooled down too fast -10℃/min. is a good cooling rate to avoid strain lines. Once the vessel has passed the transformation temperature at approx. 475℃, cooling can be faster.