You can make your own lead light cement as the materials are fairly common and safe to use. I have altered the original recipe through experience. Too much of mineral spirits dries out the mix so quickly that the linseed oil cracks early in its life. This contains the possibility of leaks through the cement. One third or less of the dryer (mineral spirits) reduces the chance of too rapid drying. This is my modified recipe.
7 parts whiting/chalk
1.5 parts boiled linseed oil
0.5 parts mineral spirits (turpentine or other)
(All of the above measured by volume)
1-2 Tablespoons lamp black or other colorant – black poster paint, or oil paint
Add the whiting (reserving about one quarter) to the linseed oil and mineral spirits. Mix this well, by hand or with a domestic mixer capable of mixing bread dough. When these are mixed thoroughly, check the consistency. It should be like molasses on a cold morning - barely fluid. At this point, add the colourant, so you will know the current colour and can adjust to make it darker.
Add more whiting as required to get the consistency you want. Experiment a little to find what suits you best. If you have to deliver the panel quickly, for example, you need to increase the proportion of mineral spirit. The mineral spirit evaporates relatively quickly, leaving a more rapidly stiff cement.
You should make only what you will be using on the current project, as the whiting separates from the linseed oil and spirit relatively quickly. The commercial cements have emulsifiers to keep the whiting from settling and so extend the life of the product. Since making your own is cheap and quick to make, there is no saving in making a lot.