silicone bronze question

[ned]

Hi there folks, this is my first post and I apologize by starting with a question.

Have recently set up small foundry for casting bronze piecespieces using investment

process, furnace and most equipment home made. Have achieved

Some good results and learnt so much in the process.

My last pour however resulted in the castings having a rose pink hue. My question is :

Can this be attributed to metal getting to hot, it did start to boil in the furnace. Using only pre used bronze without any new metal. Pouring the metal at too high a temperature ?

 

I will post info about myself the foundry and location this evening UK time.

 

Many thankfor any input forth coming.  

Regards Ned

[Frosty]

Welcome aboard Ned, glad to have you. If you'll put your general location in the header you might be surprised how many of the IFI gang live in the UK.

I'm sure there are casters here but this is more of a blacksmithing forum so don't be too disappointed if a bunch of guys don't jump in with answers. However, I don't believe you should be boiling the bronze and using all scrap is a factor as well.

Have you pickled the pieces?

Frosty The Lucky.

[ned]

Thanks Frosty, 

Color only appeared to be on surface however due to fine detail I was unable to remove. Items were part of decorative mirror. 

I hadn't meant to boil the bronze I had changed the lid of the furnace and achieved a superior melt in a faster time. The new lid really kept the heat in. I need to turn down the gas and air a bit to allow for higher efficiency. I only used all pre cast bronze as I needed a melt of 24 & 18.5 kilo and didn't have enough.

Regards

 

[Nobody Special]

I very, very much doubt you got it hot enough for literal boiling to gas. You've probably got one of a number of problems - a wet mold, dirty material, dirty crucible, or getting gas in it from some source.

Oils or dirt can form hydrogen when in contact with the metal. Your air/gas mix could be off too. Ironically, you don't want a reducing atmosphere for once. Use flux, and even better use a crucible with a lid. You also want to avoid overheating things, can cause gas absorbtion. Pour at around 2100-2200. There are professional degassers you can add as well...google will help you find them.

My favorite for this though is that you're not preheating the crucible and stock, and getting condensation, especially since you're getting a pink oxide layer.

Oh yeah, or you could have an incomplete burnout in the mold. What kind of crucible are you using? Are you using it for just the one metal? Besides the pink, are you getting any residue on the metal? Lots and lots to go wrong when casting. Might talk to Jammer, I haven't done a lot of casting in awhile. He kinda lurks at the edges and pops up on the foundry forums once in awhile.

[ThomasPowers]

Zinc coming out of a brass melt can look like it's boiling off and igniting...

[Jammer]

What kind of pre-used bronze are you using? The off color should polish out unless you seriously overheated it. Was the mold hot when you poured?

[HWooldridge]

I've cast a bit of bronze.  Couple of questions - did you use a cover over the melt such as glass? Was this cast in sand or lost wax?

[Chinobi]

I'm not hugely experienced, but I did a couple of silicon bronze casts last year that I thought I executed reasonably well and I HD a similar pink coating on the end result.  My theory is that it's a layer of copper that has ended up on the surface, kind of like it was electroplated.  It oxidized and looked exactly like copper and was removed by gentle abrasion, which at the time was actually kind of disappointing because it looked really nice!