I have several Mihec molds and started off just cleaning with hot water and dawn, and smoking them a little bit. I've been reading several of the posts about patina and lead removal and decided to check my molds with a magnifier and discovered that I also have some of the fine tinning "oxidation" and started to search for cleaning methods. I found several different posts and opinions and decided to summarize them and see if we could get a concensus on what is best before I attempt to clean up my molds and prep a couple of new ones. Here's what I've found as the generally accepted methods.
Prepping a New Mold – Clean with hot water and dawn, dry (warm up) and then heat cycle.
Heat Cycling – Remove all steel hardware except for alignment pins, heat in oven at 400° with blocks separated for 15-30 min; allow to cool to room temperature, repeat 3-4 times.
Lead Removal – Clean mold with hot water and dawn, bring mold up to casting temperature 400°, apply small amount of beeswax rub with sharpened popsicle stick, toothpick, bamboo skewer or sharpened pencil to remove lead a cleaning patch can be used to wipe the lead off.
Acetic Acid/Hydrogen Peroxide – 2:1 volume of Acetic Acid to Hydrogen Peroxide, 5% Acetic Acid (White Distilled Vinegar) and 3% Hydrogen Peroxide. The process was expected to take longer with the more dilute solution, so the brass part was immersed for 10 minutes. The results showed the same gold
color and the Lead Test swab indicated the lead had been removed. The buttery yellow gold color can be used as an indicator that the process has completed. Home Lead Test kits should be available at most hardware stores.
This procedure for removing surface lead from brass can easily be conducted at home. A 10-15 minute dunk, swirl, and rinse in a 2/1 volume ratio of 5% Acetic Acid and 3% Hydrogen Peroxide has been shown to be effective. By the way, the solution can be irritating to the skin so either wear gloves or use
tongs.
Smoking a mold – Use a propane/butane lighter not a candle, the candle wax leaves a greasy smoke. Works, but is generally considered a cover-up for other problems or until the mold naturally oxidizes.
Why brass molds get tin oxidation – “from Geargasher & Springfield” it isn't lead, or tin, that's why it won't melt away. It is accumulated oxides of mostly tin, it starts a 'seed' in a shiny spot and just builds and builds a few molecules at a time from the flash-oxidized molten surface of the metal you pour into the cavities. It's the same sort of stubborn crud that clings to the sides of a casting furnace as the alloy level drops. A reducant like beeswax or paraffin quickly takes care of it once you get the mould up to the melt point of the elemental metal.
Being an oxide, and beeswax being a reducant as JonBinGlencoe pointed out above, you can heat the mould to the point that the tin would start to melt if it weren't oxidized, apply the wax to instantly reduce the oxides to elemental, liquid metal, and wipe them away.
It happens to me with brass moulds because they do like to be run hot, it IS worse with binary lead/tin alloys, and the brass moulds DO like to be run hotter/faster than other types which DOES exacerbate the oxide accumulation.
What I do to patina a brass mould for the first session is preheat it to casting temp in a mould oven and let it cool naturally a few times, then smoke the cavities lightly with a BBQ grill lighter before ever putting lead in there. That keeps those little soldered "seeds" from ever starting. After a few casting sessions, I wipe out the soot and leave them alone. This is the only time I ever smoke moulds because it can cause a lot of other problems, but it really helps break-in with brass.
What are your thoughts?
GMT