On Fri, 20 Mar 2015 13:17:01 +1300 IVP wrote: > For about a minute, with the light just right, I saw many very > small bubbles, like a slow and gentle fizzing. If that board had > been upside-down as usual they probably would have merged > to larger bubbles that blocked the etchant getting to the copper >=20 > It stopped producing bubbles after about a minute so I took it > out, rinsed and dried it, then put it back in, this time upside-down > on the surface. Hi Joe... Yes, that is exactly what I reported in the original mail. The bubbles are so small that it's the bursting which is seen. But, as you noticed, they tend to join and then cause etching problem. I'm not a chemistry expert, but the formula of the clay (which normally is of the China clay variety when used for paper coating, a.k.a. kaolin/kaolinite) is=20 Al2Si2O5(OH)4 With a total of 9 Oxygens, I'm fairly certain that the Al and Si is bound by the FeCl and the oxygen freed. The board I made yesterday, I tried a lower temperature in the hope the clay wouldn't stick to the toner, but no luck. Hard rubbing did help a little (and the toner was also darker at those spots). Till now, the only option with clay-coated paper seems the two-step process.=20 I wonder if the Cupric Chloride etchant has the same problem. John --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .