a guide to not scratching glass

I've seen scratches on the majority of amateur made optics I've examined as well as on some professional mirrors. Below I'll describe those cases where the cause was easily identified:

• Many years ago I made a 6" Pyrex mirror using Edmund Scientific abrasives. Many fine scratches were visible upon fine grinding. The cause was identified as rust from the abrasive container. These containers were tin-coated steel. Where the tin coating was chipped off, causing rust. Small rust particles caused many fine scratches in the Pyrex. I only use MB abrasives now. No problem.
• About 6 years ago when polishing a 12.5" Pyrex mirror, many fine scratches were observed. This was traced to contamination from a brush used to coat the mirror with CeO slurry. The brush had a steel handle which had started to rust. Again rust particles caused the scratches.
• Once I was quickly making a set of 6" Pyrex optical flats for null testing optics. Didn't even build a grinding stand, so I used the wet newspaper on a flat surface technique. The resultant flats had several scratches which I attribute to not taking the normal cleanliness precautions during fine grinding.
• I once had an ATM friend whose 8" Pyrex mirror had several scratches due to insufficient edge beveling. The edge of the mirror chipped and caused big severe scratches trailing from the site of the edge chip. Several ATM books warn of this problem.

I have made about 20 optical surfaces from 13.5 inch down to 6 inch, Pyrex or crown glass. I've avoided all scratches for the vast majority by using MB abrasives and polishing agents, and by rigorous cleanliness.

The Rules

1. The grinding stand is covered with both newspaper and plastic sheets.
2. The newspaper and plastic sheets are renewed for each new abrasive grade.
3. Keep fingernails short and wash hands every time you approach or leave the grinding stand.
4. Use non-pumice, non-zeolite hand soap as these are abrasives too.
5. Take a bath and change clothes between abrasive grades.
6. Wash the optical element at least 3 times between abrasive grades.
7. Clean the wash bucket thoroughly between abrasive grades and line with a new plastic bag.
8. Vacuum the floor between abrasive grades.
9. Use a separate newspaper and plastic sheet covered table surface to lay the optical element down when you need to rest.
10. Replace these sheets as well when abrasive grades are switched.
11. Be very careful not to touch anything with abrasive-contaminated hands unless you are certain that surface will be cleaned prior to the next abrasive grade.
12. During polishing use cleanroom gloves.

If you are careful you can consistently make optics without a single tiny scratch. Once you've discovered cleanliness techniques and a manufacturer's abrasives that work consistently, stick to them.