Today I cast a tool to use to grind my 10″ mirror blank, something I haven’t done in a looong time. But it went pretty well. The one unusual thing is I only had hexagonal unfinished ceramic tiles instead of sheets in rows and columns. This turned out to be a bit of a hassle but the result was not too bad.
Basically my method is based on one Richard Schwartz taught me, first put some vegetable oil on your mirror, cover it in saran wrap, and place your tiles on the face of the mirror, business side to the mirror face.
Next put a dam around the mirror making sure it’s tight so the dental stone doesn’t escape.
Mix up some dental stone by putting a reasonable amount in a bucket and then, from a seperate bucket, add water til it’s the consistency of thick pancake batter. Make sure to get any lumps of unmoisened powder wet. The stuff I had doesn’t set all that fast so no rush. Next pour the stone onto the blank, covering your tile and making a disk about an inch or and inch and a half thick. Rinse your bucket with water from the water bucket to dilute the stone and dispose of somewhere – NOT into your plumbing unless you feel like jackhammering it out again later!!! I dump it in the corner of my lot by the tall tree on my neighbour’s side that I forlornly hope to kill by dumping all manner of noxious chemicals against (no luck so far!)
After a couple of hours the stone will be set but still feel wet – undo the dam and the tool will slide right off the blank.
Flip it over and with a scraper scrape away any extra plaster that might have gotten under a tile. At this stage the stone is still crumbly so it comes off easily. Normally I run a sharpening stone down the rows to clear out the channels but this time with the hex tiles I had to scrape the stone away in a zipzag that tool a lot longer. Once the channels are clear you can clean everything up with a stiff brush and your tool is good to go, put it aside for a week to cure.
This tool will be used as a surface against which to grind my 10″ mirror down to F/3.3 from F/4.7