Paste Soldering Tips & Tricks 

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Hello. Thank you for the question. They are referencing barrier flux.

@@RioGrande1944Is barrier flux something different from ordinary flux?

@@lizw2853yes. Regular flux is designed to encourage solder to flow. Barrier flux (also called stop flow) is designed to keep solder from flowing all over the place. Rio Grande has a video on their website about mixing your own and also sells products that is ready made.

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@@RioGrande1944 you’re welcome!!!!

Hello, thank you for the question. Here is an explanation from our tech team: When sterling is melted without protection and oxygen pickup occurs, the copper will oxidize. Copper oxide in the casting can cause two problems. First, copper oxide inclusions will occur throughout the casting. They really only cause a problem when they are near the surface, where they act as hard spots that stick above the polished surface. Second, a layer of copper oxide can form in and around shrinkage porosity. This is seen on a polished surface as gray, cloudy spots. These spots are usually too deep to be polished out. Figure 8 shows a cross section of a sterling ring; shrinkage porosity can be seen throughout. Also evident are two darker areas: one at the up- per right and the other in the lower left of the casting. These dark areas indicate copper oxide surrounding surface porosity. The presence of cuprous oxide can be confirmed with an optical microscope using polarized light. Figures 9 through 11 show the various views of sterling silver with normal illumination and with polarized light. The first photo of each sequence (9A, 10A, lIA) shows various inclusions that appear black. The second photo in each sequence (9B, 10B, IIB) shows the inclusions as bright red in color, confirming that they are cuprous oxide. In some cases, when the molten sterling is left unprotected for an ex- tended amount of time and/or is overheated, the copper oxide formation is so severe that the surface of the molten sterling develops a rather tenacious skin. This copper oxide skin greatly reduces the fluidity of the metal. The usual result is that many of the fine details on the castings do not fill. In some instances, major portions of the casting do not fill. A telltale sign of this condition is a reddish color on the surface near the incomplete fill. For further assistance regarding any casting problems, it’s advisable to contact your metal supplier.