Question: Some of the brazing paste we purchased has separated, showing a layer of liquid above the brazing material in the containers. We purchased the paste about four months ago. Does this mean that the paste is no good and should be discarded?
Answer: Please note that brazing fillet metal (BFM) paste is a suspension of heavy BFM powder in a gel-like binder system, blended together in such a way as to form a creamy paste that should be easily extrudable through a dispensing tip.
Most of today’s manufacturers of BFM paste produce paste that will remain in stable suspension for many weeks at a time. As shown in Fig. 1, the brazing powder, when mixed thoroughly into the gel binder that is used to create this paste, will create a creamy mixture that actually looks like the creamy cake icing put on top of pastry cakes/cookies.
Please be aware, however, that there are still lots of unknowns about the reactions that can occur between the many different metal-powder ions and the many ingredients in the gel-binders used today. (There are literally hundreds of different powder/binder formulations out there.) Thus, there is a high probability that every once in a while a BFM-paste end user will receive a batch/lot of BFM paste in which the BFM powder will tend to settle out from its gel binder, leaving a thick layer of liquid on top of the paste in the container prior to the promised shelf-life of that particular paste.
The reasons for this separation are difficult to determine. I have seen some lots of BFM paste remain stable for several years (in our lab testing), and I have seen other production lots of the same material from the same manufacturer separate out within weeks of their manufacturer, even though the identical production procedures were used for each lot.
Can I use, or re-store, BFM paste that has separated?
Yes. You can, and should, continue to use up brazing paste even if it separates out because the paste has NOT actually become “bad.” It has merely separated. The BFM portion of the paste is perfectly fine, and it can continue to be used until it is all gone. Merely pull the piston out from behind the paste (if it is already inside a dispensing cartridge); stir up the paste with a long spatula, spoon, etc.; and then immediately use the paste.
Please note that the paste will separate out fairly quickly once again since the binder is no longer able to hold the powder in suspension. However, I strongly advise end users to use up their brazing paste by this more-frequent stirring method rather than sending the separated paste back to your supplier for replacement. Remember, the BFM in the paste has not gone bad and can continue to be used until it is has been completely consumed.
Should I pour off the liquid on top of the paste before continuing to use it?
No! That liquid is an essential part of the BFM paste, as far as the paste viscosity is concerned, and should be mixed back into the paste before you continue to use it. If you pour off that liquid, the remaining paste may be much too thick and difficult to extrude. Always mix that separated liquid back into the paste.
Recommendation: You mentioned at the start that your paste was about four months old. I always recommend that people only purchase a maximum of 90 days of brazing paste (approximately three months of usage). There really is no justifiable reason for buying more than that. If pricing discounts apply for larger purchases, then work out a plan with your supplier in which they will only send you a month’s worth of product at a time. In that way, you can be assured of getting fresh paste for all your brazing needs.