If you buy foundation, it can contain all kinds of residues. You have no idea where it comes from. So it is wise to use your own wax. Preferably from sealing wax and / or virgin wax (completely natural and secrete it by the bees themselves). And having foundation made from it or making it yourself. By using only melted down your own old wax, you automatically get fewer residues in the foundation.
As there was hardly any 4.9 mm foundation for sale years ago and the provenance was uncertain, we then made our own comb foundation machine. Our first 4.9 mm foundation came from Sweden.
Frame with foundation 4.9 mm minimum (well) expanded.
The bottom left corner is cut off on every frame for the development of drone cells (6% of area). So many drones should contain a beehive in the season.