How to Remove Mortar From a Stained Glass Window

I ran upon some antique stained glass windows in a rehab project we were doing where they had been bricked up on the outside and were covered up with drywall on the inside.  The demo guys removed the drywall and found the windows encased inside.  I wish I would have taken a picture of them still in the wall but I was so excited to find them that I forgot.  This is where they were located (on each side of the fireplace):

When I got them out I discovered that when they bricked the window opening up on the outside they had smashed the mortar onto the stained glass windows.  This is what they looked like on the inside when I got them.  Unfortunately I wasn’t writing this blog when I did this and didn’t think to take a picture of the mortared side:

You can see the impression of the stained glass window in the mortar though:

They were really dirty and there was a bunch of mortar stuck on the back of them.  I did some research, including talking to our mason and the solution I came upon was to use what mason’s use, a diluted mixture of Muriatic acid.  I purchased it at Home Depot.  I mixed it half and half with water.

You have to be very careful with old stained glass windows because applying too much pressure to the middle of the window could possibly break part of the leaded framing.  I opted to use an old kitchen scrub brush like this one:

You have to remember that Muriatic Acid breaks up mortar, i.e. anything cement based, so don’t lay your window out on your sidewalk and then apply the acid.  Also, you’ll want to wear some old clothes because muriatic acid will eat a hole in your shirt, which it did mine. I just did it in the grass of my back yard.  I mixed the Muriatic acid and water in a plastic bucket (does anyone remember from Chemistry lab to put the water in first and then add the acid?).  Dip your brush in the acid/water mix and then begin to wet the window.  The acid will bubble when it gets on the mortar.  In areas where the mortar is thick you’ll have to gently apply acid a bunch of times and slowly the mortar will release from the concrete.  Periodically rinse the window gently with a hose to see how much you have to go and to remove the concrete particles and sand so you don’t scratch the glass.  When you are done make sure to rinse the window really well and then let it dry before accidentally dripping on your wife’s carpet 🙂  Here’s what the final product looked like:

Note that the left window has staining on the clear glass.  The mortar had etched into the window and physically removed some of the glass so I wasn’t able to remove those stains–they are actually etched into the window.  I was really please with the find and they look pretty good for 90+ year old windows.