Fixing the lens of an Artograph Tracer
The rule of thumb when repairing any kind of projection system, is that it is imperative that the lens is EXACTLY parallel and centered along the light’s primary path. If you mess it up, you fail and the projection will be fuzzy and/or distorted.

I recommend the following two step approach. Get some plasteline (non-drying modelling clay). Put the lens in the barrel like you think it should fit. Pay very close attention and center it. Roll out a 12 inch long and 1/4” or 0.8mm diameter cylinder of clay. Press it around the edge of the lens to temporarily fix it in place.

Now mount the barrel as usual and try it out. If you got the lens backwards, take it apart and do it again. Make sure you can focus it and everything with the projection is ok.

Now remove the lens and the clay. Use cold demineralised water with a drop of dish soap to remove any remnants of the clay from the barrel and the lens. Then polish the lens with a microfiber / or special lens cleaning towel.

This would be a good moment to use white cotton gloves - just make sure there are no fingerprints on the lens and that there is no dust in the barrel.

Once everything is absolutely entirely dry, install the lens as before, and now take some heat-resistant silicone and put a bead of silicon around the edge, as you did the first time with the clay. Go overboard and smear it up the sides of the barrel, but be careful not to get it on the lens. If you do get it on the lens, immediately remove it with the soapy water and microfiber. Be gentle, don’t scratch it!

Let the silicon cure (in accordance with its label) and enjoy your projector!