Preparing a mirror for coating

 

Hello again, 

 

            After doing several “Reynolds Wrap” mirror coatings, the one thing I’ve learned is evaporating Al onto the mirror surface is easy.   Cleaning the surface sufficiently to prevent water marks, pin holes, and other odd results is the HARD part J

 

            After lots of research and experimenting, I’ve finally found a system of mirror prep that works, well it works for me at least!  I break the process down into 5 categories:

1.      Removal of old coating

2.      Cleaning the mirror surface

3.      Rinsing the mirror (really important!)

4.      Drying the mirror surface (even more important!)

5.      Dust removal just prior to sealing the chamber

 

Step #1, coating removal

 

Things you will need:

  1. Muratic Acid (pool supply)
  2. Copper Sulfate (Chem. or Agriculture supply)
  3. Running Water (tap water is fine, even from a garden hose)
  4. Paper towels

 

Below is a pictorial on the process

 

 

The Chemicals

 

 

On the left is the copper sulfate and the amount of muratic acid used for stripping a single mirror.  Next is an image showing an approximate amount of copper sulfate used to create the solution (precise measurements are un-necessary, you just want to end up with a nice green solution)

 

This is our subject mirror in need of striping, re-figuring and re-coating (BTW, this is a commercial 10” F/7.7 that is ~ 1 wave over corrected, thus the need for re-figuring!)

 

Next we wet a paper towel with tap water, gently lay it on the mirror surface, then saturate it with our Green solution of muratic acid and copper sulfate.  Note the red arrow, where the paper towel wasn’t large enough to cover the entire mirror.  (Whoops J)

 

This is the same mirror after 10 minutes of exposure to the solution (Chemistry is just too damn cool IMHO J) Note the upper left edge of the mirror where the paper towel didn’t cover completely,  Properly done, multiple paper towels would have been used to cover the entire surface and edges of the mirror to remove all the original coating.  Since this mirror is going to get re-figured, I got lazy and just wanted to remove a majority of the coating {shrug}

 

If this mirror were to be re-coated, we would repeat the process, then rinse heavily with a garden hose. 

 

Cleaning the mirror

 

After striping is complete, we need to gently clean the mirror surface.  The best method I’ve found so far is to use a cotton ball and powdered precipitated Calcium Carbonate (Chem Supply). 

 

Here’s the process I use:

  1. Keep the mirror wet
  2. Sprinkle a little Calcium Carbonate on the mirror
  3. Wet a Cotton ball
  4. Using a swirling motion, start at the center of the mirror and work outwards to the edge of the mirror, gently rubbing the surface.
  5. Rinse well with a garden hose
  6. Examine surface, looking for any spots that “Bead” up (like a freshly waxed car).  If you find any spots where the water will not evenly sheet over, re-clean.  If the spots persist, use a little Potassium Hydroxide (or Ammonia), then re-treat with calcium carbonate. 
  7. Finally flood the mirror with just an insane amount of tap water!  When you think you’ve really rinsed it well, rinse it again for the same period of time, then once again for good luck! 
  8. The most important part of this whole process is to absolutely, positively prevent any of the mirror surface for drying out!

 

Drying the mirror

To be honest, this step took the longest to develop, while it appears pretty straight forward, it really kicked my ass for some time!   Here’s why, no matter how much water you use to rinse the mirror (even distilled), Trace amounts of all the stuff that’s ever been on the mirror remain.  If you ‘drip dry’ the mirror, as the water evaporates, these trace elements are redeposited onto the mirror surface and leave what’s called a “water mark”.  This watermark is invisible until the fresh aluminum hits it, then BAM it stands out like a sore thumb L

 

As an aside, I found that the latest technology for recovery latent fingerprints in forensic science is Aluminum deposition!  Yup, the very same process we use to coat mirrors is a POWERFULL tool for solving ‘Who Done It mysteries’ J  The reason this is so is due the highly reactive nature of pure Aluminum.  Aluminum doesn’t like being pure, it will react with just about everything, and this reaction will leave behind an aluminum compound that is often opaque.  Consequently trace elements in watermarks, Oils in fingerprints, and even the oxygen in air cause the Aluminum to form an aluminum compound of some sorts (Fortunately Aluminum Oxide is transparent, and pretty tough stuff so this works in our favor!)

 

So, how do you dry the surface of our mirror, with out letting any water evaporate from its surface?  I use a method called an “Air Knife”.  An air knife is simply a high-pressure stream of gas that is used to ‘Blast’ the water off the mirror surface.  Start at one edge of the mirror using a sweeping motion across the mirror until you get to the other side and your done J

 

The only question is what Gas to use?  First let me advise you that compressed air from a standard piston type air compressor isn’t going to work, the air stream has oils from the pump, as well as water vapor in it (learned that the hard way!)  The gas of choice is Dry Nitrogen, however this can be a little expensive for the initial setup (bottle, regulator, hose, and nozzle).  While I almost went this route, I looked around the shop and saw my Oxyacetylene rig in the corner and got to thinking J

 

Here I am, I’ve rolled the OA torch over to the mirror prep site, turned OFF the Acetylene and bumped the Oxygen regulator up to 20psi.  Using the smallest brazing tip I had, I ‘dried’ the mirror.  Damn that was easy J  and the coating came out pretty damn good other than a few pin holes,  Hooray !!!!

 

PS, Another method that works well as an air knife is a “Can Of Air”, used for cleaning electronic parts.  Just make sure that none of the liquid hits the mirror surface!

 

Final Prep prior to sealing the chamber

            Pinholes, Little bastards!  Seems pinholes are spawned from bits of dust on the mirror (probably freaking Fairy dust, as sometime you get them, sometimes not!)  Anyway, the best method I’ve found for getting rid of these annoying little bastards is as follows:

 

  1. Prep the chamber and load the filaments PRIOR to washing the mirror
  2. After air knifing the mirror, dry the sides and bottom with paper towels
  3. Quickly transport and load the mirror into the chamber
  4. Just prior to sealing the chamber, use a black bristled paint brush and “Dust” the mirror surface off
  5. Seal the chamber and start pumping it down!

 

 

That It!  While the process is now pretty straigt forward, I hope  you found it interesting!

 

Take Care,

James Lerch

jlerch@tampabay.rr.com