Metal Casting Zone

Hi
Woundered if anyone could help with a problem with my lost wax castings.
I have been casting some scale model parts in lost wax brass now for a few years,using the vacume assited method. All works well and have had good results, However the castings finish up with a lot of black scale on the surface, mostly all over the main sprue then up the smaller tree branches towards the castings.If the items are away from the main sprue they are clean with a nice brass finish, Items closer to the main sprue are quite dicoloured or black. Making me think it is a temp problem ??? Items do clean up in a acid bath in a ultasonic cleaner later, but it takes time. . The brass in the main sprue can have a reddish colour to it .I have considered changing the melt temp of the brass 970oC or raising the flask temp when pouring. I tryed lowering the flask temp on a previous casting and the flask didn't fill completely.
I talked to my old boss last week and he believes it maybe due to the flasks 4inch x 6 inch in size not being held at a temperature of 730oC long enough to burn out the large volume of wax in such a large flask. I burn out over 12 hours and hold the flasks at 730oC for 2 hours to 2and half hours. then cast at 620oC He has got me thinking that this could be the problem.????? Either that it is the brass alloy. Thanks Johncast

can we get a pic of the problem you are having? sounds similar to the problem ive been having that seem to be caused by moisture. On mine I can see a trail where the steam went up the outside of the casting then up the sprue to escape. the bottom of the casting was clean and shiney then switched to grey hazy as it moved upwards to escape.

Hi to all
Just a little more to add to the question I put forward earlier on this post.
I Have completed another successfull lost wax brass casting run last week.
This time the flasks 6 inch X 4 inches were burnout over 12 hours as usuall but at the end of the burnout cycle the flasks were held at 730oC for a full 4 hours this time then dropped to casting temp of 620oC. My results were very good. But still a little redish/black stain on 2 of the 3 casting trees.
Now the tree of model componets that cast very well and with very little or no black stain or scale was of a smaller volume than the other 2 ,at 130 grams of brass.
The other 2 castings trees were of the volume of 330 grams quite a lot heavier .My question is. Does the volume of wax comtained in the tree, have a bearing on the burnout time required for the flask. The 4 hour burnout at 730oC was just right for the 3rd flask that cast with very little black stain,but not long enough for the other 2 flasks due to the higher volume of wax, another hour at 730oc for these flasks could of been needed.??????
Or I am completly on the wrong path here and the black stain on the two castings is purly from the higher temp in the mould due to their higher volume of brass.
I hope I could post some photos and will try later. Give me casting any day over computers.

Im not the most experienced lost wax caster there is by far, but one thing i have noticed is that the burn out seems to be more related to the thickness of the investment more so than anything. it seems to me that thick investments take longer to burn out due the moisture it retains, this causes the water inside to turn to steam and actually keep the investment temp down until it is driven out.

Question, wich ones did you pour first? my previous thoughts dont seem to apply to your problem as the one with a higher wax volume (less investment) came out ok.
Next time you cast try pouring the first little bit off into a clean ingot mold and the same with the last bit to see if there is any crusties on it once it cools, that way you can eliminate possible contamination..... just a thought

is the casting sound?? can the stain and scale be removed with phospheric acid (rust converter)?????im thinking it may be wax residue and just the colour that the brass flashes upon cooling and the cast item is actually ok????

Hi
Thanks Aone and Xl for your input ,Xl you are pretty spot on with the black scale ,the castings are all quite sound but, tarnished black, but do clean, You mentioned the brass coming in contact with wax residue ,This is exactly what I have been thinking,and there shoud be no wax residue at all in the flask when the burnout is complete. Longer time at 730oC for bulker trees. and I haven't tryed rust converter ,what a good idea.My business partner does some brass etching and I have been using some of his copper persulphate soluition in a beaker in my ultasonic cleaner to clean the castings and depending on the strength this can clean them up ok but a lot of time and work.Last time the solution didn't work at all because it was to old. Must try the rust converter.

If this file goes through OK there are two photos attatched of lost wax brass castings of model parts I did last year with the staining problem I am having , look close into the centre of the large sprue and the black /redish stain may be visible. Some stain is visible in the sprue butt.These were all usable castings but took time to clean up. I want a good casting with a good brass colour.
PHOTOS DIDN"T SEND WILL REDUCE FILE SIZE AND TRY AGAIN

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Haooray Horay John downloaded a photo

Hi to all I have edited one of my previous threads on the question on the problem of the fire scale on my brass castings and have now a photo of a casting ,have a look at the shot and if there is any suggestions as to the cause of this red/brown/black residue I would appreciate any help. The colour stains did clean up ok to a brass colour later when cleaned in some mild copperpurslphate in a ultrasonic cleaner.. But the surface of the brass is very pittered ,under a eye glass the surface looks like cracked paint on these castings. I 'm thinking the burnout temp wasn't spot on 730oc for the last four hours of burnout.. or a better Alloy with more Zinc is required, rather than scrap sheet brass.

that looks like the muck i get when i use borax for flux, i quit useing it and started pickling my brass before melting.I soak my scrap brass for about 15 mins and then brush with a brass bristle brush, the brass turns to a coper color but is very clean when melting. Try the pickle and see if it works for you

I have been doing a lot of reading to narrow down the areas I need to change to get a better result ,One of my last castings that I have posted into my profile page cast very very well with little staining. What was different ????TEMP?? Im wondering about flask temp, needs to be lower. metal temp,higher 1020oc. One section in my book mentioned not to add borax to graphite crucibles (interesting) Don't stir brass alloys, which I have done both of these. Another area is that I have been quenching the flask into a bucket of water possibly far to soon after casting and the flask need to cool more before quenching?????Could this quenching cause the sudden cooling of the metal and give the poor pitted surface texture on the brass.
I'm contemplating experimenting with 2 more flasks, cast one at low temp 500oc with high temp metal around 1020oc and add no flux agent, wait a long time before quenching the flask, see the results, then make adjustments if needed for the second flask. I will keep researching possibilities. fraustrated Johncast