How to Flatten Glass With Corks

Most of my work is flattened so I'm well-aquainted with corks--most specifically the smell of them burning. I'm generally using corks to flattening vessels that end up between 15-26" tall x 10-16" wide x 3-4" deep, so medium to larger work. Corks aren't the only tool to flatten glass, some other materials you can use and their pros and cons are:

• Metal - leaves chill marks, sucks heat quickly, can stick if hot, not good for soft curves, heavy for a large hand tool
• Wood - doesn't suck heat quickly but can stick and/or leave scum, can stick, not good for soft curves, burns out quickly
• Kiln shelf - used waxed it can be good for crisp-flatness, can leave chill marks or texture, can be sticky if not maintained, fragile and heavy for a hand tool
• Graphite - similar properties to metal but generally smoother on the glass, extremely brittle and fragile for a hand tool
• Newspaper - doesn't suck heat out of the glass, great for curves, burns out quickly, lacks good handles so potentially risky
• Cork - doesn't suck heat out of the glass, can create varying degrees of flatness and soft curves, doesn't stick if you keep it moving, long lasting, smooth on glass, lightweight so good hand tool

Years ago I bought some of the commercially available cork paddles but found they lacked good handles and were only available with rounded edges, so I began making my own.  I discovered there were economies in buying giant cork slabs and making a dozen at a time so I could have whatever sizes I wanted and I could sell the extras to other glassblowers.  Selling them generated some questions about their proper use from buyers who hadn't used corks before.   

Here's how I use them:

Before using a brand new set I burn them in by lightly charring the surface in front of either the glory hole, open furnace door or with a fluffy torch--this gives them a thin layer of black carbon.  This carbon layer will touch the glass and help them slide nicely rather than stick (kind of like a block).

Some people like to soak corks before using them, but after years of using them totally dry I do mist mine at the beginning of a piece.  I'm sure totally wet corks will last a bit longer, but there's a risk of scarring the glass wet cork and it will have a greater cooling effect when in contact with the glass.  Using them moist but not wet, I run less risk of scarring the glass and the glass still moves nicely while you're using them. The wetter you use them the longer they should last but I'd rather burn out corks than risk scarring the glass with dripping wet corks.  As they burn out quickest in the center I occasionally do have to sand the corks flat again, but I generally get 2-3 years out of a set (and I flatten almost everything).  I also make my own corks so I'm not that concerned about longevity.

I generally start flattening vessels while my assistant is capping the pipe--this maintains the air volume and prevents the piece from denting or caving in.  To start, my assistant will first take a strong, even heat. Once he's back at the bench, I'll have him stop turning where I want the edget to be.  The instant he stops turning and just before the glass begin to sag on the pipe, I rub the bubble between the paddles in a circular motion.  Since the glass is sagging, I can only rub for about a second before my assistant needs to flip the piece and I flatten again. We keep doing this--cap and flatten, flip, cap and flatten, etc. until it's as flat as I desire.  If it's really tough to flatten a piece, get it hotter.  If it's getting too wide or if I want it extra flat, I'll have my assistant uncap for a second--this will let the vessel flatten quickly under the corks without internal pressure pushing back. Uncapping gets things flatter quickly but the glass will usually dent, so either uncap only very briefly or give it a quick puff to re-inflate and re-flatten. I move quickly and firmly on the glass with the corks and keep them moving constantly, this prevents the cork from sticking and imparting texture to the glass.  It probably helps them live a long life as well. The paddles always smoke and occasionally flame if I take too long. They don't seem to mind.  I usually get the flatness I desire in one heat, ocassionally two.  If you find your glass sagging at the neck too much during this process, hit the neckline briefly with some compressed air for a second or two while turning just before flattening.  

The corks eventually will begin to burn out--usually in the center.  This will create a dished effect on your corks; when this dishing gets deep enough to become a problem, I'll sand them down on a belt sander to re-flatten them, then re-char them.  However, some people prefer this dished shape and look forward to their corks burning out a bit.  If I find the corks feeling a little 'tacky' (usually after 2 years or so) I'll also sand them a bit remove a layer of carbon.  To preserve the life of the corks, right after using them I generally put them down flat on the floor to extinguish them.  I've seen people put them down in a cookie sheet with a layer of water to extinguish them, but the water seems to make some of the carbon flake, so I'm not confident this really extends their life.