Why Steelite plates have pin marks.
Pin marks occur during the manufacturing process.
After glazing, plates are fired through the glost kiln, which fires the glaze. To ensure an even and complete glaze coverage, plates are placed into "cranks", supported under their rim on three small refractory pins.
After firing, a small amount of the refractory may remain in the fired glaze layer, this residue is removed by grinding to leave a smooth finish.
Occasionally, the grinding process may result in a small area of the underlying body becoming exposed. This is not detrimental to the performance of the product since the biscuit fired body is fully vitrified. Vitrified products are defined as having zero porosity and, as such, will not stain or become unhygienic when in use.
The alternative to the use of pins is to support the product under its foot. To do this the glaze must be removed from the whole of the foot prior to glost firing. This is quite acceptable for products destined for use in the home, where the frequency of use is low and a greater degree of care is exercised in handling crockery.
For commercial crockery, foot-placing flatware items have serious negative implications for product performance; the large area of exposed biscuit body remaining around the foot will result in problems due to foot-to-face abrasion of the glaze surface when plates are stacked together.
A fully glazed plate back should help to ensure a long, trouble-free service for the life of the product.