This sanitary ware tank lid was made in China. Notice how thick the white glaze is being applied to cover the iron containing body below. This is a testament to how opaque a zircon opacified glaze can be. Of course, high percentages create a stiffer glaze melt and conditions can more often combine to produce crawling like this.
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.
A type of porcelain zircon-glazed ceramic that includes bathtubs, sinks, toilets, etc.