Wednesday, 26 April 2017

Make Your Own Stopping Knife

“Stopping knife” is a traditional term for an oyster knife with a weighted end.  This makes it a multi-purpose tool that manipulate glass, dress lead came, act as a fid, act as a putty knife, and become a hammer.  It also stands up on its own.  I find it the single most useful too in leaded glass panel construction.

This note is how to get from here:

To here:

The process relies on the low melting temperature of lead.  This means that you can use stiff paper wrapped around the handle of the knife to contain the molten lead until it cools.

First you set the oyster knife into a vice and cut two dovetail joints at right angles to each other into the end of the wood handle.  This will insure the lead is firmly grasped by the wood and will not come loose during use.

I do this with a fine bladed saw such as a hacksaw, coping saw or even a dovetail saw.  There are Japanese saws that work very well too, but are not so widely available.

The top of the dovetail joint should be just a millimetre or two off centre. 

The angle should be about 30 degrees from vertical.  Saw down far enough to get a 6mm chisel into the space between the two angled cuts.

Chisel out the wood between the cuts.

Repeat for the second dovetail at right angles to the first.

Now you are ready to prepare the oyster knife to become the stopping knife.

Use paper of more than 90 grams per square metre, such as cartridge paper to form the narrow cone.  Set the knife at a slight angle on the paper. 

Secure the beginning edge to the knife handle with a bit of masking tape.  Mark the paper 5 mm – 10 mm above the top of the handle.  This will be the fill indicator when pouring the lead.  If you over-fill the cone, the stopping knife will be heavy and uncomfortable to use.

Roll the paper around the handle to form the cone.  This cone should be as close to vertical as possible.  A wide based cone will, of course, provide stability, but it will add so much weight as to be uncomfortable to use.  It will also be so wide as be uncomfortable for the palm of your hand.

You can unwrap the paper and start over if the cone becomes too wide.  The key is to start the wrapping just before the handle begins to taper toward the end of the handle.  The other way of looking at it is to attach the paper just as the expanding taper stops.

Try to keep the paper cone as smooth as possible.  This will form the shape of the lead end of the handle.  You want it to be as circular as possible without dents or angles.

Now you are ready for the casting.

I use a small old cast iron pot to melt the lead.  I place this over a camping gas burner to provide the heat.  I promise that I did straighten the stabilising legs before lighting the camping burner.

Put some old lead came into the pot to be melted.  While this is coming up to heat, place your wrapped oyster knife in a vice with heat resisting materials around the site to catch any spills.

Put sufficient lead into the pot, as there will be impurities floating on top and the lead will cool quickly when taken off the heat.  The photo below shows the amount of lead used.  This 100mm diameter pot has lead barely covering the bottom.  You do need enough lead to complete the pour at one go, as a second pouring will not stick to the first adequately.

The photo shows the last piece of came just about to be melted.  This is the time to begin the pour.  If the lead is too hot, it burns the wood creating gases and multiple bubbles splashing hot lead and leaving an unpleasant surface for the tool.  As the last piece of the came melts and leaves its impression as the piece on the left, it is time to pour.

Pour at a steady rate into the paper cone until you reach the height indicator you previously marked in the paper.  When you stop pouring, set the pot on a heat proof surface.  You will notice some smoke and browning of the paper.  That is normal.  This picture shows the effect of the hot lead on the paper once the smoking has finished.

This photo shows the inside of the cone while cooling.  The cooling process will take about an hour.  You will be able to check, by touching the paper, how hot the whole is. 

When the whole is cool, you can unwrap the paper from the handle.

This shows the roughness of the handle end.  This is due to the bubbling from the scorching of the wood and paper.

When the paper is removed and the lead is fully at room temperature you can use a rough file to remove the bubbling and to round the edge of the lead.

The oyster knife has been transformed into a stopping knife and is ready to use.