Making a Magpie

Discussion in 'Lost wax casting' started by DJN Holistic, Aug 29, 2022.

  1. DJN Holistic

    DJN Holistic Silver

    I'm starting to plan out how to sprue up the main body of the Magpie, but I could do with some advice as I'm dealing with a few concepts I'm not too familiar with, those being working around a hollow core and having a gate that spilts. I'd love some feedback on this plan.

    I've no included the cross section as it's dependant on how this angle works.

    White Line: Hollow section
    Red Line: Thickest section
    Solid Red: Cup
    Solid Orange: Gate
    Solid Yellow: Vent

  2. Mantrid

    Mantrid Silver

    I think that will be fine. This is how I sprued up my owl and it is similar in that the main sprue goes to the body. The wings cast really well despite being so thin (you can see through the wax in places). The cooling of the metal will start at the wings and they will take metal from the thicker body which in turn take metal from the thicker main sprue and to there from the thickest part, the pouring cup. I cant see anything wrong with your design





    Attached Files:

    Last edited: May 6, 2023
    Tops and DJN Holistic like this.
  3. DJN Holistic

    DJN Holistic Silver

    Tail cast went okay. A little bit of shrinkage which I might be able to chase out it so minor. A small section of one of the spines didn't quite fill. I'm not sure if that's even a casting defect or a bit I missed when I was chasing the wax. Those spines have been a headache since the clay stage. Because I imbedded some thin wire into the clay to form the spines, they lifted off the surface slightly when I was bending the clay to get the shape I liked. Silicone got under wire when moulding it and it resulted in multiple little gaps in the spines which I had to patch up every time I cast a wax. Given how fragile the tail is as wax, I've broken, recast and patched at least six of them and it got pretty tedious.

  4. DJN Holistic

    DJN Holistic Silver

    Body sprue done. Looks okay to me so long as it's tilted slightly to position the legs as close to the bottom as possible. I was a little concerned that the feet wouldn't fill, so I webbed the toes to simplify the shape somewhat. It's gonna mean a lot of grinding to carve the toes out, but better that than missing feet. Pinned the core with some sil-bronze rods and added a bit of patching wax around them to prevent any ring indents around the puncture point. One last pass to clean it up and patch any marks, then I think I'm ready to start dipping.

    BodySprueBack.jpg BodySprueSide.jpg
  5. Mantrid

    Mantrid Silver

    Great job. I was going to use bronze rod for my core pins in a piece i did but I was concernedcthat maybe they would melt or soften at the shell firing stage resulting in the core moving. So to be on the safe side i decided to use small steel nails instead. Looking forward to seeing the casting.
  6. Mantrid

    Mantrid Silver

    Another thing you could do is to have the core protruding out of the wax so when you apply your shell it is also gripped by the shell at both ends. If you do that you probably wont need the pins atall
    Sorry I didnt see the second pic that the head was there. So you wouldnt be able to lock the core into the shell at both ends. But then even at just one end may be enough if it goes into the shell a decent amount
    Last edited: May 11, 2023
  7. DJN Holistic

    DJN Holistic Silver

    I'm a bit worried that they might melt too. The wax is less than 1cm thick, so I'm gambling on it cooling quick enough and the exposed section of core being enough to anchor everything in place. Fingers crossed :/
  8. DJN Holistic

    DJN Holistic Silver

    Started the burn out and had a bit of a disaster. The shell shattered along the ridge. The piece was too large to dip, so I brushed the slurry on, working one side, then the other. I think this might have caused a seamline which weakened it's integrity. Not sure if it can be salvaged as it's not just a crack, but several chunks that are completely detached. I'll try to cement the pieces back into place if I can. There's no wax in the shell at the moment, but it's unfired. Break.jpg
  9. Mantrid

    Mantrid Silver

    Put them in place as accurately as you can and paint some thickned slurry over the crack, sprinkle with stucco allow to dry and repeat a few times more. Then do the firing.

    When I did my owl it broke off the rod while I was still applying the shell. I cut back the shell and rejoined the wax and applied fresh slurry and stucco to link the remaining shell and it was fine. You may have a bit of extra work to do chasing the bronze but better than starting from scratch.



  10. DJN Holistic

    DJN Holistic Silver

    Uff, I take it you dropped that?

    I've started patching the shell already. It seems to be going okay. Not sure how I'm going to fire it though as it's too big for my little bin. I have some fire wool, so I might have to make a single use furnace. Nothing fancy, just roll it into a tube and fashion a lid. It only has to hold together for 20 min or so. Guess it might be time to upgrade my burn-out equipment. That'll be my next project I think.
  11. DJN Holistic

    DJN Holistic Silver

    Casting failed :( It'd be quicker to say what went right!

    So when I was pouring, there was a hell of a lot of turbulence. The metal was boiling and erupting from the gate like a volcano. I'm not sure what the cause of that was, but I suspect it was gas escaping from the core as the metal hit it. I think the turbulence was a big factor in a lot of the mess this ended up being, but not all.


    There are pot marks on the back which could be from shrinkage or caused by the turbulence.


    The feet didn't fill properly. I knew from the start they were going to be a problem, but dared to hope.


    The metal froze before it could fill the wings completely. It they also lack definition and look half melted. I'd put that down to inadequate venting, but since the metal froze before even reaching the vents in some places, I'm not sure what to think about that.


    And then there's the head which is missing half it's skull. A venting issue I suspect, not helped at all by the core releasing gas and the fact that as the eruption was going on it sealed off the vent on the gates side.


    I think I'm going to have to revert to my original plan of casting the wings and body separately. I've no doubt I'd have any problems doing it that way. I might have to do the legs separately too if they are going to be such a hassle, although how I'd weld them on is a bit of a conundrum.
  12. Mantrid

    Mantrid Silver

    Thats a shame.
    Im not sure about the vent failiure in the head.
    The top of the head filled. I think a vent failiure wouldnt have allowed the metl to get that far up.
    Theres alot of cruddy looking bronze where the neck is. It suggest to me a possible beakdown of the core and blocking of the channels with the bit.
    Where is the pouring cup and main sprues?
    If think I would give it another try but have the pouring cup at the tail end instead. Then the metal has a straight run to the head and not have to turn the corner and flow upwards to a similar level as the pouring cup where the pressure forcing it upwards would be reduced.
    Other things to try is to just have the ceramic shell flow into the cavity instead of a separate core. It was quite large and my not have been fully water dry.
    Other possibilities may be some wax still present in the head.
    The wings are not too bad, maybe hotter bronze or hotter shell would resolve this.
    In saying all that you are going to have to do welding anyway so maybe doing the wings seperately is the safer option.
  13. DJN Holistic

    DJN Holistic Silver

    Yeah, I think I'm going to ditch the core next time, although if you take the violent reaction out of the equation, at least it held together and the pins kept it in place. Silver linings... *shrug

    I think if I cut the belly section away from the body, it'll open it up enough to to allow the shell inside. Having it separate will let me cast the legs as their own simpler piece too. If I cut along the bottom of where the wings attach and where the colour change of the birds markings happens at the neck, it should disguise the seam, at least in theory. Making sure all the parts fit together correctly is going to be tricky though. fSzl0k5CTi-5ge8bTuRRPQ.jpg
  14. Mantrid

    Mantrid Silver

    I think a ceramic shell core will be fine without going through such drastic measures. Just pour into the current hole. If your not sure about that you could still use the pins in conjunction
  15. DJN Holistic

    DJN Holistic Silver

    After a bit of thought, I think I'm going to give casting it in one piece another go before resorting to cutting it into pieces.

    First issue to solve is making sure the core is properly fired to avoid another disaster. I'm in the process building a kiln more fit to task, so hopefully that'll solve that issue. I've got a hold of some cast iron grills, a sack of fire cement, and have some insulation bricks on order to put something together.

    Addressing the casting defects and how to fix them for the next attempt...

    1: The missing head. While it's the largest defect, I'm chalking this up to the vent getting sealed. As seen in the picture, the vent was plugged at the wrong end. A thicker vent might be prudent too. I used a 5mm, but maybe something closer to a 10mm would be safer. Otherwise, there doesn't seem to be any real need for a change in design here.


    2: The wings not filling completely. This was purely a case of the metal freezing. I should be able to fix it by just having everything hotter. Hotter metal, hotter shell. I'm not sure whether it'll make much of a difference, but I think I'll double the number of vents running from the wings too which should reducing the air pressure a tad.

    3: The feet not filling. It was my concern that the issue with the feet was that they wouldn't fill properly before freezing. However, looking at them now, that doesn't seem to have been an issue. I'd hoped that by webbing the toes, it would have been a simpler shape to fill, but what I think happened is that the metal flowed straight through the foot, down through the vent and that trapped air within the rhombus shape (pic below). So rather than webbing the toes, it's probably better to sprue each tow individually. Freezing is still an issue, but if I'm heating everything that bit extra to ensure the wings fill, that that should work to my advantage here too.


    4: Shrinkage on the back. How much of this is from the core explosion and how much is from shrinkage is a little hard to tell, but there's defiantly some significant shrinkage going on. I guess the first step is to step up the size of the gate and try to pull the reservoir closer. Moving the entry point further towards the back of the shoulder can't hurt either. But honestly, I find controlling shrinkage to be a bit of a mystery, so I fully expect to have to do some repairs here. To that end, I've simplified the detail on the replacement wax to make it a bit easier to do repair work on.


    Mantrid likes this.
  16. Zapins

    Zapins Gold

    Lots to unpack here.

    I agree hotter bronze will help. Looks like it froze before making it everywhere. Don't necessarily want the shell too hot tho or it leaves a hard to remove burnt /bad finish.

    Easiest way to fix this is to cast a hollow core without a floating core. I agree the core degassing or shifting was likely a major failing here. Tig welding is the way to go, though oxyacetylene or even a kitted out arc welder or mig can do the repair. If you don't have a welder then I'd advise getting one. It will make these kinds of repairs a cake walk rather than relying on getting a perfect cast (which can be done but is often the result of dozens of trial and error attempts to figure out the best system which is usually only valuable in a production run setting not for 1 off pieces).

    The shrinkage issue happens in the back because shrinkage happens at the junction of a fast and slow cooling piece or between a very thick and very thin piece. The wings are both thinner and have a large exposed surface area compared to the shoulder region which is insulated and likely slightly thicker. Having an open hollow core instead of a suspended closed one will largely fix this issue as it will even out the thickness and help let heat escape evenly across all parts. The insulated core stays molten longer than the exposed wings which cool and shrink and draw metal from the center.

    While I think your current spruing setup can work, with a better/more evenly coated shell that is stronger Turbulence can be reduced by making a J shaped sprue and bottom feeding the piece. This let's the turbulence settle out inside the main sprue and provides an extremely even and non turbulent filling of the piece you have fixed to the upturned tip of the J sprue.

    Your piece has a lot of detail in the feathers which would need to be recarved if you cast parts separately so I'd suggest trying to cast it as 2 pieces body wings and head together leave the belly open and weld back the belly onto the final piece. Legs may also be worth casting separately as hey are thin and delicate.
  17. DJN Holistic

    DJN Holistic Silver

    I do want to get a Tig, but I don't have the money for one at the moment. The one I have my eyes on is and R-Tech one for just shy of £600. I've seen ones around £300, but I'm not sure whether I trust them to work, even if they claim to have good specs and functionality. I have some art work going into a fairly high profile show at the end of the month, so fingers crossed I'll sell something and be able to buy one, but I'm not holding my breath. So in the meanwhile I'm stuck using my oxy/propane torch. It'll join stuff together but there are a few issues. First it takes a long time to get things joined and holding the parts in place while I work is tricky because I can't really tack them together easily. Second, it's not a very clean job, especially when welding along a line as the filler tends to want to flow together into a single blob like the liquid terminator reforming in the movie. So I'm trying to keep my limitations in mind with how I separate the pieces.

    Do you think it's possible to insulate the wings somehow and control the rate of cooling? Thicker shell? Wrap them in fire blanket? Or am I thinking too far outside the box.
  18. Zapins

    Zapins Gold

    Yes wrapping wings would be another option or burying mold in bucket of sand with chest exposed to air can help aevwn out temps and make it more stable while pouring. But will still have some small effect of shrinkage with a solid chest. The easiest way to avoid shrink is to make sure the bronze is cast as thin as possible and still allow complete filling so 1/8" to 1/4" is a good thickness to shoot for and be aware of areas that may be more insulated.

    What Flux are you using? It's been a while since I oxy acetylene welded but I think 20 mule team borax (the laundry detergent) is a good silicon bronze flux. It helps prevent that balling up and allows bits to easily flow/join together.

    Check out some second hand machines. Usually they work for many years without too much issue - especially the older heavy ones. Arc welders (the cheap tomb stone/stick welders) can be used to tig weld, you just have to get a compatible tig torch and argon gas tank and then scratch start the arc as they don't have a high frequency start that will initiate an arc without the tungsten tip contacting the bronze. May be more affordable until you can save for a nice new setup.

    To improve the shell you can get a very large wide tub and pour shell over the whole piece so it coats evenly instead of brushing it on.

Share This Page