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"Purple/Yellow discoloration on surface with Pewter 92" Topic

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RFCasting17 Feb 2017 5:15 a.m. PST

First time ever I've started to get a purple/yellow discoloration on the surface of both my alloy in the pot and on the surface of my casts… any idea how to solve this? Or what it is?
Ive got various conflicting answers i.e: lead contamination or tin oxidation?

For info: Ive not changed any settings on my melter, or running my moulds any differently. The last batch – from the same supplier – was silver in the pot and the casts. As soon as I started feeding bars from this batch in the surface of the bot became discoloured (yellow, through to purple the longer I let it build up), and it isnt a 'dross' layer that could be scooped off. I just have to use a bottom feeding ladel and ignore it for the most part.

Only been casting two-three years so still not that knowledgeable about metals.

Is this just something that will eventually burn off, or are all my casts from this batch of metal going to be discoloured? Or worse is this going to cause serious problems with the miniatures?

Rich Bliss17 Feb 2017 5:55 a.m. PST

I can give you better advice if could post a picture. It sounds like a thin oxide layer that might be the result of higher humidity.

RFCasting17 Feb 2017 6:20 a.m. PST

Easier said than done – no matter how I set the white balance its just making everything look silver: try this?
The tank was cast from my last batch of metal, its silver/grey, the troopers were cast today, to the naked eye they are distinctly yellow/silver.

Taking a picture of my pot is impossible, the camera just shows it as silver, but to the naked eye the surface is distinctly purple. I could let a bit cure on the back of the ladle and photograph that I guess?

Im not so much concerned with what it is so much as how I fix it, but the two answers are inseparable anyway I guess.

Rich Bliss17 Feb 2017 8:00 a.m. PST

Yes, the two answers should always be inseperable. 😀

It does look like an oxide/sulfide layer. The color is due to refraction so it's very thin.
I'd suggest playing with you pot temperature a little bit. It may be running cooler now than when you first started using it. Try increasing the temp slightly and see if it effect the outcome

Master Caster17 Feb 2017 8:28 a.m. PST

From your first entry input you make it very clear the yellowing started with the introduction of the second batch from your supplier. The key question is is this new batch turning out bad castings other than the color? I can't find anything in either of my two shop sources about the yellow coloring being a contamination unless it inhibits the casting process and the resulting pieces.
If you try to buff the color out with a metal polisher and it goes away on the surface you're probably looking at some sort of oxidation. If the color is throughout the metal then something else is going on here.
A blue and/ or purple color is indicative of the alloy being too hot too hot for the alloy currently in your pot. Either your supplier changed the alloy mix just a bit more or less of a certain metal will change the alloy's properties, or some form of contamination was introduced into this batch. Have you notified your supplier of this concern?
Do you regularly flux the metal in your pot (sal Ammoniac is a standard but don't breath the fumes. Unass the area)?
Do you regularly introduce fresh alloy ingots? If you continually run your pot at a high temperature and just keep introducing cast mold sprues then you will slowly but surely end up with metal that has lost a lot of its positive casting attributes. This can happen if you continually cast small items and don't cast any large items that use up higher quantities of alloy.
Oxidation could be from iron from the sides of your pot and ladle.
Oxidation could also be introduced from your molds and/or residue from whatever the masters were made from that formed your molds. Or you could have thrown an odd piece of metal that was unknowingly had some zinc, copper or an undesirable metaloid material. Zinc is a killer and just a small bit will contaminate an entire 100 pound pot.
If I find more on what could be causing the distinct yellow coloring I will come back on here.
(BTW, photographing with sunlight or regular incandescent bulbs will cast a yellowish color to the white metal castings. Try special 'Daylight' bulbs and close the curtains from outside light. Oh what a difference!)

RFCasting17 Feb 2017 10:10 a.m. PST

The colour depends on how 'thick' I allow the surface to get, so if I let it build up a while and get some onto the back of the ladle, its purple. For the sake of the castings its thin enough that its a yellow tint.

'Fluxing' your pot is something Ive never heard of?
Ive tought myself almost entirely via Google and books (which are a mixed bag, full of materials and equipment advice thats outdated by three decades, but very useful for mouldmaking advice!)

I was aware that as Im casting small objects and regularly melting back down the sprue and cores the metal in the pot would get worse with the build up of contaminants etc, so every now and then I try to use up everything, empty it, wire-brush the dross out out, and re-fill. I do go through a 50kg box of ingots every two months or so, so not sure how many Im feeding into it.

As for the castings, they are fine, just tinted yellow on the surface. It does 'feel' different though. I pour at either 290-300 for 6mm, or 280-290 for 28mm, depending on model detail and how warm the mould is. This new pot (I say new, its maybe 10% the old bars) just doesnt feel as 'viscous' as before…

For the time being Im going to empty it into an ingot mould and re-fill. I have a big sized job to get cast and I dont fancy them all being yellow :(

Random Die Roll Supporting Member of TMP17 Feb 2017 10:18 a.m. PST

Speaking from a production point of view…all of the information above points to the problems…but your information from above "starts off yellow and moves thru purple"---I am thinking that you have used this melter for some time now---so feeding in too much material is probably not it
I would lean toward a problem with your melter that just happened at the time you added the new material. I have seen melters that are gas and electric, so without knowing what you have, I am just going to say that you have a cold spot, and it sounds like the temp control is working ok, causing other areas of the "pot" to become overheated.

The Man With Two Bryans17 Feb 2017 10:45 a.m. PST

Check your new batch of metal is actually Pewter 92. I assume from the name it is from AIM/GWN, so it should be stamped 92/7 followed by the batch number, eg 92/7 2850. The boxes should also be clearly labelled, although sometimes the delivery note gets slapped over the details. Suppliers aren't infallible: I've known of one missupplied batch of metal before.

RFCasting02 Mar 2017 7:05 a.m. PST

Checked all my bars that I hadnt melted – all 92, two different batches by the look of it. Yes I use AIM/GWN

(does anyone from GWN still work for them now? I'd avoided saying who's alloy it was incase I was accused of devolving the thread into 'industry gossip' haha)

So I emptied the last of the pot out into an ingot mould (the ingots are actually gold coloured now rather than silver!) wirebrushed my put to get the last crap out, refilled – from the second box of the batch – and that was fine.

Still leaves me with one big 1.5kg 'gold' bar that I'll either never use, or use as a last resort. I guess a contaminant got in there somehow, but I'll never know whether it was me or a single bar from the first box of this shipment.

Pretty irritating, but just goes to show you cant 'dilute' down a pot once you get something in it. I have resin and all sorts in the same workspace so I'll be a damn sight more cautious and tidy now just incase it was me.

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