Attached to the side of the anvil (enclume de cannonier) was a small device. In the film, the smith's regularly scape the heated material on it. I believe they are using it to scrape slag. There is no evidence in the film of fluxing. They never show it being applied, nor is there any container nearby to hold it.
I think Steve has nailed it regarding the silicone. This became an issue when coke was introduced for the smelting. The coke introduced a high silicone content. The Belgian barrel smiths complained of this as they felt it affected the look of the finished product. For a time, they went so far as to import wrought iron from Sweden.
Eventually they got Cockerill to produce runs for their needs.
Pete
