Wednesday, September 8, 2010

Pennsic House: Windows

Every house should have windows and since mine was a 16th century house, they needed to be 16th century ones. After looking at many paintings, bull’s-eye glass seemed to right choice. Bull’s eyes or Roundels were the "leftover" centers of the large discs of glass blown to create small panes of glass. The panes were cut from the outer part of the disc and the centers were trimmed an used as filler or to create “Bull’s-eye” panes.

Here is the image featuring a Bull’s-eye window I decided to replicate.

Since I knew I was not going to use real glass in the windows as they would be to fragile and heavy I chose to vacuum form them from clear PETG plastic. In order to keep the detail of the lead came and glass surfaces, I would use a “female” pattern. This means I would be using the surface of the plastic that comes in contact with the mold. First I isolated the window from the painting and mirrored it so to create a complete window. I then scaled it based on what I thought the real size of the window was (based on items in the painting) and on what size would look good in the house.

A friend, Jason Klein is an accomplished glass blower and owner of Historic Glass works and he offered to create the glass bulls eyes (or roundels) for me. As we had become good friends at Pennsic, we both thought this would be a nice collaboration. I received the roundels from him and I made RTV silicone molds of them. From these molds I cast urethane plastic copies that I could then cut down to the needed diameter as well as cut the in between “triangles”. Using real lead came I fabricated on complete “Pane”. Since I’m not an experience solder and even with a period “Iron” I could not get proper (read neat looking) joints. Well since I needed to clay up the spaces between the plastic roundels and the came to keep mold material out, I also clayed up proper looking joints.

Here is the finished “Master” ready to be molded.

After placing a metal frame the same size as my vacuum form table around the master, I used what we call Bondo-resin to coat the master. This coating is made by taking Bondo, (a micro-balloon filled polyester resin used as a auto body filler) and adding polyester resin in order to thin the mixture to the desired viscosity. Once the Bondo-resin has hardened I filled the back with urethane foam. I turned over the master and repeated the process on the other side. Once both sides had cured, I removed them from the master. The resulting 2 pieces are negative impressions of the bulls-eye glass pane master, one female vacuum form pattern of each side. These are placed into my small vacuum former and after heating a sheet of .062” PETG (A clear thermoplastic) I lower the frame onto the “platen” and apply a vacuum. [Note: Do not think of a vacuum as pulling or “sucking” it is more like when you pull a book out from beneath a stack of books. The other books are not “sucked” to the table, they merely fall into the hole you create. Same with a vacuum, there is roughly 62 miles (100km) of atmosphere over your head, think of it as an air ocean that you are at the bottom of, and if you could put a 1”x 1” x 62 mile tall column of it on a scale it weights 14.7 lbs at sea level. So when you create a vacuum, your making a hole and all that “fluid” we call air, flows into that hole and if something is in the way, it will push down on it with a maximum of 14 lbs per square inch.] So with a flick of a switch the machine makes a hole and the air flattens the plastic against the Bondo-resin pattern.
Here it is in video:

Once all the pieces are vacuum formed (2 per window x 6 windows) The pieces are rough trimmed and then glued back to back with an industrial hot glue gun. With the addition of small plastic balls glued evenly through the pane to space the two sheets the proper distance from each other, the assembly is placed between a layer of soft foam and ¾” plywood and clamped. Then I fill the void between the sheets with polyester resin. (The small plastic balls keep the sheets from “crushing” together between the foam) If you do this, be aware of the amount of MEKP (catalyst) you use. As polyester is an exothermic plastic (generates heat when curing) it can get very hot and soften or melt the PETG surface.

The piece still have their blue protective vinyl film, this was deliberate (normally these are remover before forming) because I now cut around all the “came” with an X-Acto knife and peel off the protective vinyl covering the lead came area, leaving the film over the Roundels. Mixing up a grey paint, then adding some silver metallic powder I have the “lead came” paint and can now paint the came.

Once the paint dries, I remove the remaining blue vinyl and “ta - da”, a realistic looking bulls-eye glass pane. (In one tough lightweight piece of plastic)
I took one of the first test panels and cut out a section and made a mock up of what the window frame and casement would be.

Here is a cross section of that mockup

The frames are made from surplus mahogany “Fireplace surround” kits it be sold off cheap at a local home improvement store and with a few passes through the table saw they have been reprofiled to work perfectly as 16th century window frames and casements.

In the source painting there are these latches to hold the panes into the casements. It took me a while to figure out what these looked like, but then I found a high resolution copy of the painting and could make out what they likely were.

Here is the brass master I created for these latches.
I made a vulcanized rubber mold of this master for my 12” spin caster and cast 60 of them in pewter. At some point I’ll cast these in brass so they will be stronger, but for the sake of speed and economy, I chose pewter as I could do it cheap and fast myself. Each cast “screw” had a steel wood screw cast in and the heads were designed to work with my 8 point sockets so they would be easy to install. Once they were all cast (50 of them) I had them antique /blackened brass plated.

To complete the look, I added steel bars across each pane. Later this year I will most likely add the small pull rings shown in the painting.

The finished windows separate into an outside casement and an inside casement and are screwed together trough the walls using ¼-20 stainless Allen screws. (I know another tool to bring, It’s up to 3 now!)

Here are the finished windows.


  1. Chris, thanks for doing this blog. It's really inspiring, and it offers great insight into a completely different way of doing things. your description of how a vacuum table works is simple and brilliant. Thanks!

  2. You’re welcome. Thanks for commenting.

  3. Those are very artistic windows - from ashes to classic. Historic glass windows are pretty rare nowadays. Congratulations on making your own. If I were to have them, I'd likely keep then inside as a keepsake.