On the blog of the coineys' favourite Expert Collector of Celtic Art FSA, we find the following misleading statement "the Celts could not solder or weld". I suppose he thinks they used superglue or spit. Nevertheless smiths in the Iron Age , whether 'celtic' or not formed objects by hammer welding of separate components (see for example Radomir Pleiner, The Celtic Sword, Oxford: Clarendon Press 1993). Indeed, turning an iron bloom into bar stock always involved the process.If smiths could not weld, they'd have nothing to work with when producing objects like the 'fire dogs'.
Back on the old continent, the PAS have handled more iron age objects (whether or not made by 'celts', who knows) than the antiquarian fellow has had hot breakfasts. They see evidence of solder on them (here or here and here for example). But here's a thing, while the describing FLO says in each of these cases that there is something which "might be" solder, no analyses whatsoever are done to verify that. These objects then disappear into the pocket of a collector and the skimpy description made with the finder hovering over to get his stuff back so, after giving the landowner half its value, it can be added to his private object-fondling-hoard, or put on eBay. This is certainly no way to "advance archaeological research".