Light loads on initial run-in will allow combustion to pass by the rings. This escaping combustion burns & oxidises the oil & bore fragments forming a chemical coating on the honed bore. This chemical coating is known as "Glazing" and has the appearance of golden varnish with the original hone finish being very visibly. Once an engine is glazed up it is usually impossible to achieve complete bed-in of rings. The glazed coating prevents the ring to bore contact required to match up or bed-in rings.