A scratch on the surface of the glass generally is caused by contact with metal or glass. The scratch may be straight, curved or jagged. Often, the depth will vary throughout the scratch. The depth of the scratch will determine the quality of the restoration.
CIRCULAR LIGHT SCRATCHES
Glass can be easily scratched during the cleaning process. A plastic scrubby can even create these scratches on soft glass. Razor blades and other scrapers can cause hairline scratches as well. These scratches are generally not deep but may cover a large area.
When glass rubs against glass it will scuff the surface. The glass is white where the contact was made and may cover small or large areas, depending on the amount of contact.
HARD WATER STAINS
Hard water, when left to dry on the glass, can leave behind mineral residue as the water evaporates. The buildup of mineral deposits eventually becomes visible, making the glass look dirty and crusty. The hard water stains appear as white or opaque circles on the glass.
Many buildings are washed with a soap and acid mixture to more effectively remove grime and debris. When the acid mixture is allowed to dry on the surface of the glass it will lightly etch the glass which interferes with the clarity of the glass. The acid staining appears to be dried soap suds on the glass that won't rinse off.
The surface of glass is rough and uneven when viewed under a microscope. These irregularities trap dirt and other debris that adds up to inhibit clear viewing through the glass. The glass appears dingy and less than clear and sparkling.