Nearly all glass painting begins with the application of tracing black, that is, the black lines in the drawing. They are important to offset the lead lines that will be added later when the glass pieces are assembled. If the tracing black is too delicate, the painting will look wan (though this may be the effect desired in some cases), and it is too bold, it may overwhelm and make the details appear too aggressive.
There are many varieties of tracing black available from Reusche, hightlighting tracing how tracing black is a cornerstone of the art; Stencil Black, Universal Black, Tracing Black #61, Soft Black, Bistre Black, etc, each with its own set of subtle differences. For instance, "Tracing Black #61" is often considered the all-purpose traditional "standard" but Stencil Black is especially shiny, dark, fine-textured and has a good range of shading when used for matting (you can use your tracing black color for matting). It's my personal favorite, and that of many others. For preparing the paint for tracing black, you need to have the following supplies on hand:
Mixing the paint
Place two tablespoons of your chosen tracing black powder on the float glass (it is transparent and cannot be seen in the photograph, but it is there!), and add a sprinkling of gum arabic. The gum arabic serves two purposes. The first is the make the paint smooth and viscous. The second, more important rose is to allow the powder particles to stick to each other and the glass when dry. Without it, the paint could be brushed off completely with too little effort.
Add water or table vinegar and mix with the painter's knife until creamy. Some paints can remain granular for a long time and require long, patient mixing. Tracing black usually mixes very well. If you have poured too much powder, keep it for later after it's dried up, simply re-moisten it with water next time you need paint. Choose table vinegar if you want to do the matting immediately after, and skip a firing. I like to use water, and fire the glass before I matte - I haven't yet tried the vinegar method yet.
Applying the paint
Make sure that the glass you will paint on is very clean. When you start applying paint with the liner, do so atop a light box, and make sure that your lines are thick enough to block all light, but not lumpy. If the lines are lumpy, the paint will have an unattractive crackled appearance after firing. Make the lines slightly wider than they need to be; once the paint is dry, you will scratch off the excess, and that's where your work as an artist begins. The tracing black step is little more than the straight application of the paint on the glass. VERY IMPORTANT! Once dry, you cannot re-touch the paint. You have to do each area in one go, and while you may contact a still-wet area with your current application, you must avoid contacting a dry area. Once finished, expect your piece to look very coarse, and be disappointed - that is normal.
Glass paints as applied to float glass can hardly be attractive, because the glass is so smooth, it doesn't react, or interact with the paint. Glass is a "dead" support for paint. The paint just sits there, very flat, and it is in the "scratching off" stage that you will be able to give it a "graphic arts" quality.
Below is a detail of the tracing black BEFORE scratching off the excess. Not attractive at all.
Scratch off all excess
It is now that the coarse lines are going to be refined. Using the sharpened back end of a paintbrush (paintbrushes are ideal as they tend to be made of fine-grained wood), remove, very carefully, all excess paint. With this technique, you can achieve very fine lines, very straight lines, very sharp lines. Concentrate, and take your time. This step is critical.
You can see here the same detail, after cleaning up:
You may right-click on the image below to print it in larger format and examine it closer.
Tracing black is fired at temperatures varying between 1180-1250F.