Metal Casting Zone

Honestly I have no clue as to what you have going on now, I thought for sure it was flux on the part. I have a hard time beleaving its the flask temp, I had no way to measure my flask temp before i added the temp controller to my kiln and didnt have the staining you get. I think im going to have to go cast something myself and see if i was just lucky on a few castings or if it might be the alloy you are useing. The brass i have been useing is from old sand cast parts, not extrusions. I use just tap water also but the investment says not to use tap water and to use either DI or RO water, but our water here is pretty soft compaired to most places.

I have never casted Brass, but I have casted quit a bit of Bronze (similar enough : ). The Bronze I use is a silicon based bronze alloyed for casting. the Silicon additive helps to prevent Firescale and staining and increase flow. my casting temp. on this are 1980*F - 2010*F (1082*C - 1098*C) depending on light or heavy pieces. With Flask Temp. at 1050*F - 1120*F ( 565*C - 605*C) By comparison I would say your temp. look pretty similar.

My Hunch : ) Thanks for posting the picture. I am not rulling out going with a silicon based Bronze alloy. But, my hunch says you have Hot spots in the mold, which is allways the case with electric kilns. if the flask in placed in the corner of an electric kiln the two sides laying against the Heating Coils will be hotter then the two sides facing away. Another reason i use silicon based Silver and Bronze to avoid this problem. Hot spots cause firescale and color and alloy problems. however I use a Large gas kiln which is more friendly to cleaner burnout and more even dispersement of heat. i do use electric kiln on smaller orders sometimes. Try placing the mold in the center of the kiln and possibly pulling flask and letting it sit 3-4 minutes outside the kiln and then cast. I have used this procedure on occasion.

Hot spots really show up on large flat pieces such as the piece you are casting here. so extra care is to be taken.

normally the rule is go flask temp cooler. but i am going to say go hotter then let sit 3-4 minutes outside before cast. Oh Flask should fire at casting temp. for 1 hr per flask inch. general rule. so if you are using a 4 inch flask then it should stay at casting set point for 4 hrs for the mold to completely reach even through-out temp. i usually do not go 4 hrs and i use 4 inch. 2-3 but give that a try on flat pieces and un fluxed alloy.

Jonathan

Hi Jonathan
Thank you for such a in depth answer to what my problem could be. Much appreciated.
The area regard the hot spots is completly new one, I have never read about or heard about this in 30 years in the jewellery trade. I would not rule this out ,however the many brass castings I have done in the last 6 years would, to me, indicate a flask with a very even heat throughout,not just in spots. On examining the castings trees after cleaning all show good brass colour at the ends of the items ,furthest from the main sprue,the closer the item is to the main sprue the blacker the casting becomes.look at my profile page pics and you can see.
I once cast a tree at a lower flask temp and it faled to fill,so have been reluctent to cast with reduced flask temp,but I think this happened because the melt temp was far too low at 950oC. I intend to cast next with a melt of 1012oC and a flask temp of ,as you mentioned 560OC or so and see any difference..
You mentioned the time for the flask in the furnace to come back to casting temp . after the flasks have been at 732oc for 4 hours I reduce the temp to the casting temp I require. 620oC ,and allow it to sit at this temp for 1 hour to equallize, then cast ,are you saying that the flasks should sit at this casting temp for 4 hours for a four inch flask before casting. Could be a point to think about, my furnace is not that big ,maybe it should sit for 2 hours then cast. My flasks could be very much hotter than the temp indicated on the furnace. (I see the point) .
The possibility to change to a bronze alloy may be the easier answer rather than this brass alloy or scrap alloy as I am using. But cannot find anyone that supplies brass casting alloy here in Aust. let alone bronze. This is the next point of examination I think. Thanks again for your help. JOHNCAST

Hi everyone

Have completed another lost wax brass casting run last night. Complete experimental run with all parameters the same as the last run. ONLY this time the flask casting temp was very much lower 500oC for the first flask,not 620oC, big items still cast with good detail but copper colour throught the casting and fire scale. The second flask was poured at 450oC and to my surprise the small items cast with very good detail and a nice brass colour all over ,That solves the small items casting perameters. the larger items are better ,less fire scale than the items cast at 500oC but still have copper dicolouration throught the surface. (Similar to the casting on the right in the photo in my earlier post in this thread)
This lastest experiment begs questions.
Is the brass scrap alloy I use never going to work?? If not why does it cast a small item perfect and a large item in the same flask with fire scale and a copper discolouration.

Does the flask need to be even poured cooler at 400oC or 430oC for the big items?
Should all the brass alloy be new metal?
Should I change to a bronze alloy?

Any ideas would be much appreciated thanks
JOHNCAST

My understanding is that you pour into a scalding hot flask, wait for the mold to cool then check the casting. I also noticed that you said when you used the lower temperature that your casting came out better. Have you tried a room temp flask? Also might there be an issue with the flask material burning when the metal hits it? Any impurities?

Hi xarlock667

Thanks for your input. It has been suggested I do as you said and wait for the flask to cool then cast However I think this practice could introduce a difficult parameter ,regard the temprature of the flask when it is poured. the flask would cool at different rates given the outside temp at the time.My boss took the flasks from the furnace to the casting arm with no waiting time. To cast our men's dress rings.
I feel a room temp flask would to my knowledge be a very dangerous thing to attempt,molten metal pored into a cold flask could react as it does if poured into a cold metal ingot mould , the shock of molten metal onto stone cold metal makes the metal bubble and spit, throwing of fragments of alloy into the air. My boss still has the burns to this day after forgetting to heat a ingot mould before pouring his gold. I understand sand casting is done into a room temp mould, but the sand mould can vent air differently than investment. The other aspect would be the flask wouldn't allow the metal to flow into the flask to fill the castings. Most of the casting processes I have seen all require heated moulds. As for impurities,this could be the issue and I require a different brass alloy. Thanks for your help JOHNCAST

I have known about lost wax casting since my dad first told me Ruger used it in their barrel making process about 20 years ago, and I am about to try it myself. I understand you make a wax copy of what you are making then coat it in the casting medium, then melt out the wax, then pour your metal. My question is what is your casting medium?

The reason I ask is that I have gotten contradictory reports, and experimentation has proven them to be full of misinformation. I was told that Plaster of Paris would explode if used even if cooked for several hours first to dry out the mold... Yet it is part of my refractory compound and that has held up to temperatures that damn near melted my steel crucible. (No explosions yet.) Mind you, I dried the hell out of it before use.

If the issue is that you are worried about the cast, or flask exploding, then perhaps you could switch the casting medium? I do not actually recommend plaster, as it smells like hell and discolors the metal, but perhaps a Porcelain or ceramic coat? While room temperature is out for these materials, it would prevent outside contaminants from tarnishing your castings. That would leave your metal, and your crucible as potential candidates, and rule out your casting medium.

If that is NOT it, then you have 2 options left:
1) The scrap you are melting is failing, or impure.
By failing I mean seperating into its component metals. Perhaps stir it a little? It also might have been contaminated before you ever got it, as it is scrap. If you have some of known quality, you might melt it and note the result.

2) Is there something in your crucible that is contaminating your metal? Never mind I just reread the title and feel like a moron. Your crucible is graphite, and therefore pure. Damn.

Well I hope I have helped in some way.

Hi xarlock 667

Thanks for reply. You have highlighted a few points here.I think the point on the alloy is number 1 on my list. scrap metal or the brass I use is cut from sheet brass and brass rod, a few sections from other castings run but the old metal is never more than 40% of the total melt. have done a cast with total new brass and still got poor results, but this was done at 620oC flask temp.

Your other point on the alloy seperating is quite possible, having castings that have reddish tinge through them and other sections a nice brass colour. However I have been stiring the alloy once it has melted a minite before it is poured.. Maybe the alloy should be stired more than I am doing. Interesting point.

The point I refured to in the last post on the flask exploding is not in regard to the investment medium it is due to the temperature difference between metal and flask.
The information on the plaster of paris may be correct. On it's own this is not a medium to be casting metal into.
Investment plaster is made of a type of plaster or plaster of paris of some kind but has 3 to 4 other materials incorporated into this for high temp casting.

I wouldn't use any material for this lost wax casting that wasen't manufactured for that job. My boss cast with Kerr Satin 20 for the jewellery we made and I am also using this same investment for my casting. It is possible I require a different type of investment for brass as there are several in the range with different properties and this could be one of my problems. Casting lost wax is a difficult process even for the experienced,so to add another possible area for compilcations by making your own investment is one we can do without..Any jewellery supply place can supply investment for casting. Thanks again for your help JOHNCAST

HI again

Another casting run to recast the items in the previous photos. all the parameters were the same as before,only the casting flask was lower temp at 435oC to 440oC. Brass was melted at 1015oC no flux ,stired.
I am very pleased with the results as far as the detail of the model parts,yet again ,the reddish pink tinge of copper is all through the larger castings. Small items don't have this discolouration ,quite a good brass colour.

What could be causing the brass alloy to change to this colour pattern. It looks as though the mix of alloys in the brass seperate when melted and poured. Even though the brass is stirred in the crucible within 15 seconds of being poured.
Would a brass of 70/30 be a better alloy for casting?

Again any help would be appreciated Thanks

Wonderful pics.
Job well done. Would love to see more!