On Spark plugs insulator, it is covered round a transparent glaze (glaze just like glass), so the insulator looks very clean and clear. On insulator, we always, print the spark plugs number and company logo..,on insulator. in the world, for the printing on insulator, always using two process method., 1. over -glazing, 2, under-glazing.

Over glazing and under-glazing
Over-glazing means the insulator glazing first, then printing the words on the insulator. In fact, the printing is on the glaze, not really on the insulator .So we call this method as over-glazing.
Under-glazing means, the insulator the fist printing , after printing then glazing.
so it is easy to understand, that over-glazing, the pigment of printing is easy to be drop by knocked or washed. But it is impossible happed on the under-glazing.

Why under-glazing is difficult to adopt in spark plugs line.
Printing process looks easy, but put the under-glazing technology to adopt in the spark plugs line, it is difficult. Because of spark plugs process need very high temperature. Assemble the insulator (which must be glazed), terminal, inner seal material, and centre -electrode need under very high temperature. ( AGA' s process temperature is 1000C. So if you use the under-glazing method, the pint pigment also will be under the high temperature. Because before the glazing, you must printing the words on insulator.
Over glazing is not, after the insulator glaze, they will be Assemble with the terminal, inner seal material and centre-electrode. After that, printing words on the insulator. So the pigment is not under high temperature.
In the world, the normally industry pigment, at 600C will be nothing. So under the 1000C still have color, the pigment need special . it is very difficult to got these pigment material for anyone.
AGA's under-glazing pigment, the row material is bought from Germany. Confected pigment is by ourselves. The pigment color is few, only blue, black, and sky blue, and Red. Because it is difficult to make these pigment. The temperature is so high.

The advantage of under-glazing
1.Spark plugs looks very nice.
2.the pigment can not drop by bear rub and oil contamination. because the print pigment is in the insulator, not on it.
3.Can print very small words on insulator
4. It is the good way to as under-glazing as imitation-proof, and client will easy to know which spark plug are copy, which spark plugs are original . Because the almost of maker of spark plugs copies, is the small factory, they do not have ability to got or make the pigment to Under-glazing. in the world, only few company can make these pigment and it is not for sale.