Reproducing Pattern Glass for Leaded/Stained Glass Restoration/Repair

Sometimes when restoring older leaded glass windows we need to replace antique pattern glass which can no longer be sourced. In this tutorial I will be demonstrating the use of “castable refractory” as a permanent mold material for reproducing old pattern glass using a ceramics or glass kiln. Castable refractories are powdered ingredients which are formulated for use in high-temperature applications such as smelters and cement plants. When mixed with water the material behaves much like wet cement but able to withstand 3000 degrees f. The material is typically not yet used in the kiln glass world but does indeed have many applications as a more resilient alternative to plaster in mold making and kiln casting.  I have produced several tutorial videos covering this material in use for various kiln glass projects, here's the link to the main list: https://theglassfoundry.com/mark-lauckner-glass-tutorials/

Replacement glass, Signboard Coro-plast material or similar, Modeling clay or similar, Silicone mold release, 3000f. Castable refractory mold material, Kiln shelf or bisque fired ceramic tile, shelf primer kiln board release. Tools: kiln, glass cutter, spoon, mixing bowl, pail.

I’ll start by removing and cleaning the example of pattern glass I would like to reproduce. I am going to place some modelling clay or plastic signboard material around the perimeter just slightly lower in height than the glass thickness. This will provide even edges which will be easier to cut off the finished piece later. Cracked pieces can carefully be fit together and the modelling clay helps to secure them in position.

Now the sides need to be built up to contain the cement casting. I usually make the sides 3/4" wider than the piece of glass I am going to reproduce. I usually use the Coro-last signboard material for this and tape it together like a rectangular box about an inch high.

Next I am going to sift the refractory mix through a kitchen colander to remove the larger pea-sized pieces to produce a finer detail. I mix the refractory powder with just enough water to make it fall off a spoon when held on it’s side. Too much water can weaken the mix, too little water can cause uneven setting. Before placing the mold material, I spray the glass lightly with a silicone mold release which oils and “wets” the surface which makes the material flow better with less potential to trap air bubbles in the small areas of the design. 

The castable refractory I use for this sort of mold making is the 3000f dense castable and it is available through refractory suppliers which have an outlet in every major city and industrial town. The cost is comparable to plaster, approx $5 for a $50 lb sack. It does have a shelf-life and absorbs moisture so it is recommended to buy it from a supplier, not an online reseller dispensed into smaller quantities of unknown storage conditions or dates.

After filling the mold with approx. 3/4 inch of the castable material, I vibrate it by pressing the back of the spoon down into the wet mix until I can feel it knocking the glass, repeatedly and over the entire surface. This forces the heavier material under any trapped air bubbles.  

I set it aside to harden for 24 hrs. Then I can strip off the dam sides and carefully remove the glass. Now the mold needs to air-cure for at least 5 days before firing to 1400f at 300/hr. After the mold is fired and cooled I can examine it to see if there are any areas where I need to gently scrape off thin lines caused by the fitting of cracked pieces, etc.

 Now I pour a thin mix of shelf primer over the mold as a release so the glass won't stick. Pouring instead of painting the thinly-mixed shelf primer ensures there are no brush strokes left in the mold.  

Now it is ready for my replacement glass. In this example I am using a different (darker) color of green because I would like to indicate which is the original and which is the copy. I cut a piece of glass at least 1/2 inch wider than my molded piece and place it on the mold. Then I place a piece of primed kiln shelf or bisque-fired ceramic tile on top of it. This bit of weight prevents the edges from pulling in and thickening and it also helps to press the glass down into the design areas of the mold without becoming elevated by trapped air.  I fire these to a full 1500f to ensure the glass has taken up all the fine detail. If the glass didn't have a flat surface sitting overtop of it, the glass would tend to pool up on the mold during the hot firing. The little bit of weight helps the glass stay spread out and slowly forces it down into the fine detail.

(A word about “press-firing”. Too much weight contributes extra mass to the firing and will lengthen the time required to reach useful temperature, because all that extra mass also needs to become just as hot as the mold and the glass. Also, the extra mass can result in uneven cooling because it will hold more heat and longer as the surrounding mold and kiln cools. The glass doesn’t need much weight at all, it will press down and conform to the mold details when it reaches the melting point. The work being done is more a result of time and temperature than pressure.)

If the press-firing has left a noticeably dull surface on the flat side of my glass, I will sometimes opt to bring it up to just 1400f again with the pressing board removed. This will ensure the glass surface is re-melted flat and shiny if it is required. It is an extra kiln-firing step and sometimes it can be avoided, especially if the design is quite complicated or the window is a distance away from close view.

The finished replacement piece now needs to be cut down to the size of the original before fitting into the window. Back in step-1 when I attached raised ares around the original sample I was preparing for this cutting. Those raised mold edges ensure that my glass is of even thickness in the region where it needs to be cut.

Which one is the original and which one is the copy? The darker green is the copy.