The 4mm thick tesserae I sell on the website are hand guided so I cannot guarantee they will be all exactly 4mm, some may be 5mm though I work to avoid this. I continue to improve the cutting templates I use but I have to be able to change them over and then ensure that they are placed correctly and don’t move. As you can see from the picture there isn’t a lot of room for error.
If any rods come out too thick or too thin then they get thrown into the scraps box but some will always get through.
Other companies who supply the thin tesserae have computer guided cutting saws which is why they charge nearly £30 per kg, I’ve even seen one charging £70/kg for 4mm thick.
I use a small grindstone and rub it over the finished mosaic to take down any edges. and this does the job, but just be aware that, although I do my best to avoid any variation there may be some.