Mirror Discoloration

Tips for Using Mirrors in Stained Glass

Mirrors can be used very creatively in copper foiled stained glass work. You can use them as a regular looking glass, or incorporate them into lamps or panels. All you have to do differently is make sure you treat the mirror pieces with edge sealant. Here are some useful tips on how to use them and how to avoid mirror discolouration and damaging the backing.

A stained glass mirror with a wave on it.

Cutting Mirrors for Stained Glass

  • To avoid scratching the silvered back of the mirror place it on a towel or something soft
  • Make a score in the same way as you would regular glass. Do this on the top of the clear glass not on the backing. NOTE: Mirrors are often 4mm deep, not 3mm.

It’s very easy to chip the backing when you run the score. To minimise this, do the following:

  • Break the mirror immediately after scoring
  • Use running pliers from both ends to open the score first
  • Intricate curves can also be a problem. Press with your thumbs or the padded handles of your pliers rather than tap the score. If you do decide to tap to open the score do it very gently.

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Grinding Mirrors

  • Silver backing chips very easily so use the finest grit grinder head you can get – super fine if possible. Alternatively, use an old grinder head which is dulled already. If you use a coarse grit it will chip into the mirror backing
  • Keep the backing uppermost to stop any scratching on the grinder bed
  • Apply gentle steady pressure as you grind
  • Wash and dry the mirror

Before Copper Foiling the Mirror

Have you noticed those black blobs appearing like you get on old glass mirrors? Sometimes this happens a while after completing the piece.

  • You need to seal the mirror along the edge where the backing ends to prevent the backing becoming damaged by flux during soldering and to stop these black blobs from creeping in on your piece.
  • There are several products you can buy that do this. They are called silver protectors or mirror edge sealant, such as this  CSL mirror spray* (paid). Try spraying it on a Qtip to apply it neatly
  • Alternatively you can use nail varnish, standard varnish or shellac. These do the job just as well but they’re messier to use and you don’t want it going everywhere.
  • Just paint neatly along the edges of the glass, sealing the backing to the glass.
  • You can also spray the whole of the back of the mirror with a clear sealant like the acrylic sprays use for cars – or hairspray! You don’t have to do this but it does give added protection to the mirror backing.

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Copper Foiling Mirror

  • Let the sealant dry
  • Copper foil over the dried sealant as you would regular glass


  • When you solder, the backing is less resistant to heat so go faster. Alternatively, use less heat with a cooler iron
  • Mirrors can scratch very easily if you use steel wool on your solder. I use an index card to cover the mirror back if I use steel wool.

What is First Surface Mirror?

first surface mirror or front surface mirror (also commonly abbreviated FS Mirror or FSM) is a mirror with the reflective surface being above a backing, as opposed to the conventional, second surface mirror with the reflective surface behind a transparent substrate such as glass or acrylic.

We in the stained glass world use FSM for making kaleidescopes.

Questions from Readers

rust colored

Rust coloured mirror discolouration with copper foil by Deborah

I have tried sealing the mirror pieces after they are cut – on both the backs and the sides – but something is still effecting it.Is the flux leaching into the mirrors in my stained glass pieces? If so, is there a kind that won’t discolour the mirror? Or is it the rinsing that does it? or cutting with oil on the cutter?

Would love some advice!

Milly’s reply:

Hi Deb, thanks for your question and photos – it’s helpful for me to see what’s happening.

These spots are very common – sometimes a rust colour but most commonly black. It’s caused when the the reflective metal coating that backs mirrors becomes tarnished. Whenever mirror is cut, it creates a weak spot where contaminants – such as flux – can creep under the coating.

To give yourself a fighting chance, try to make sure you choose an acid-free flux acid-free flux.

But the secret is sealing the edges tightly with a mirror edge sealant. You can get this from most stained glass suppliers, eg Delphi

Question: How do I clean the sealant off the glass after it is sprayed all around. Will this harm the glass?


It’s best to be very careful and not get the sealant on the front of the mirror as it’s hard to get off. Spray it on to a Qtip first to avoid this happening.
Try a non-ammonia cleaner or Isopropyl Alcohol, but be careful at the edges as you don’t want to take it off the bit that’s protecting the back.

Reader’s Mirror Tip

I have used a good quality nail varnish when I’ve run out of sealant. It’s important to spray on both the back and front edges of mirror. Pay particular attention to the places where the backing is chipped, as this is where the corrosives do their nasty work!

I put my mirrors on a brick in a large shallow box, and spray around each edge, turning the box around. Two coats is more effective than one if you can stand it. Do this before leading or copper foiling.

Cutting Stained Glass Circles

How To Copper Foil Stained Glass

Tips for using mirrors in stained glass.

23 thoughts on “Mirror Discoloration”

  1. I put a piece of Contact paper on the back, usually wood grain or clear, and grind it with a fine grit mirror side up. I make sure the foil or lead come over the contact. I try to use as little flux as possible. I have had no trouble in about 40 years.

    • No, I’m afraid there isn’t Joanne. The sealant is the key; make it impenetrable and you’ll be fine for your next project.

    • No, I would still protect the edges when leading as the discolouration will creep out beyond the lead into the panel.
      Good question, thanks for asking Diane.

  2. Hi Milly. Thank you so much for ALL your texts. . I love them. They are informative & keep me enthused.
    I have used Yacht varnish to brush all over the back of my cut mirror pieces. It is simple to use & I’ve had no problems so far. A little more expensive but sure your pieces are worth it.

  3. a very good source of First Surface Mirror

    is vintage photocopier

    like the xerox 2000 have a big one inside about 3\8 stick and 4×10 inxh a great value
    also cannon vintage have some
    go to a dealer and asking for recycling mirror inside about 1 or 2 inch by 10 inxh long
    the vintage photocopier gestener whit draving move over have x6 pieces inside
    a mine of treasorus

  4. Hi Milly, I look forward to these Sunday emails every week as they are so informative and inspirational especially during this pandemic when I am not going out much at all. Thank-you very much.

  5. I am starting a project with mirrored stained glass and referenced this before cutting. It’s not my first work with mirrors but I couldn’t remember what worked best for me in the past. Now I know on which side to make my cuts and which side up while grinding. I will get that old worn out grinder tip out (knew I was saving it for something, somewhere, sometime). Your tips of placing on a towel and pressing, starting running lines, etc. will be most helpful. Thanks! I have sealed with mirror sealant in the past with success. I sprayed the entire back and edges.

  6. Milly, your tip on working with mirrors was great, especially for beginners. I have had good success sealing the edges first, then spray painting (saves a lot of time) the entire back of the mirror. This does take time to dry before foiling, ( I wait until the next day) but has worked well for me over the years.

    Thanks for your willingness to share.

  7. This is a nice article and solves an issue that I have seen and tried to deal in the past. Keep the good work going, I enjoy each week’s notes


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