Subsurface Shrinkage Beneath the Gate

Discussion in 'Lost wax casting' started by DJN Holistic, Jul 22, 2022.

  1. DJN Holistic

    DJN Holistic Silver

    I've been running into an issue on and off. What looks like a good cast at first glance is hiding a shrinkage defect directly beneath the gate, only revealing itself after the sprue is removed. I believe it is called "macro porosity".

    What I think is happening is the metal in the gate is solidifying faster than the metal in the pattern or the pour cup, effectively nullifying the effect of the shrinkage reservoir. With no metal to pull on while cooling, the metal shrinks beneath the gate, causing it to tear.

    I'm unsure how to work around this issue, and the frequency of it occurring is quite random. For example, with the sparrows pictured below, only the two pictured showed this defect, while three others were fine. All five were cast at the same time and were identically sprued.

    Red lines in pics show the location and shape of the gates.
     

    Attached Files:

  2. Mantrid

    Mantrid Silver

    How are they cast? Sand, lost wax?
    Pictures of cast with sprues vents etc would give more clues
    Sorry ignore the question
    just realised its in the lost wax section
     
  3. DJN Holistic

    DJN Holistic Silver

    I don't have any pictures of the sparrows, but here's what the bull looked like sprued up.
     

    Attached Files:

  4. HT1

    HT1 Gold Banner Member

    something for scale would be nice,
    but yes shrinkage is leaving a hole, you have to feed that, so a larger tapered gate could help, i know in the world of art casting inverting and pouring out the excess metal to make a hollow casting is very common, but I have no idea how it is done, safely

    if it is an option you might consider changing alloys to something that does not shrink as much that would probably come out in a bronze, or at least be repairable,
    I have a piece( very thick Plaque) that can only be done in bronze never brass or it will get shrinkage defects


    V/r HT1
     
  5. Mantrid

    Mantrid Silver

    If the bull is solid then that would explain it. Normally the casting should be hollow unless it is small and the reservoir of the pouring cup is larger than the sculpture.
     
  6. DJN Holistic

    DJN Holistic Silver

    This makes sense for the bull, which is solid and weighs around 3kg, but it doesn't explain why the same happened with the sparrows which only weigh around 200g and are roughly equal in size to the pouring cup.

    It's also strange that this only occurs specifically at the gate and not at the vents. Looking at the bull, the large vent on it's butt is at the same elevation as the gate, meaning the metal there would be cooling at the same rate. So why didn't the same shrinkage happen there too, or at any other vent for that matter?
     
  7. DJN Holistic

    DJN Holistic Silver

    Here a picture of the sparrow sprue.
     

    Attached Files:

  8. Mantrid

    Mantrid Silver

    The cooling rate would also be related to the thickness of the shell or investment around the bronze. Where the sparrow is theres more mass of bronze in this area. I think looking at it the pouring cup and sprue would be less insulated and would have cooled faster than the bird. As it solidified faster its shrinking would pull the sprue away from the birds head. Then when the bird cooled and shrunk the was no bronze to flow down into the head from the sprue.
    I think I would have poured the bird belly up with a very short sprue attached there and a larger pouring cup, and at this orientation you wouldnt have needed vents as the air would not get trapped anywhere.
    As for why the vents didnt shrink away from the bird possibly as they cooled they were pulling bronze from the bird which was still liquid, adding to the shrinkage at th birds head.
    This is all theory and could be completely wrong. But I have learnt to keep the walls of the sculpture as thin as you can make them to avoid shrinkage problems.
    In the bull I would have tried to make it 5mm or less.
    I read somewhere that the quality of a bronze sculpture is meaured in how thin it is rather than how thick.
     
    Last edited: Jul 23, 2022
  9. DJN Holistic

    DJN Holistic Silver

    My first attempt was belly first, but it didn't work very well and I got a ton of porosity defects. The shape is almost like a banana, so pouring in one end and venting out the other made more sense, rather than inverting the shape and pouring into the centre of the arch.

    Isn't this always going to be the case though? Any pattern > gate > cup design is going to be shaped like an hour glass. I like the idea of insulating the gate however.

    There's a bronze artist who lives a town over from me. He's well established and I went to have a chat with him a while ago. His advice was to cast solid pieces whenever possible, simply because people like the heft they and they sell better on average. The mad man was casting a life sized orangutan at the time. Soild, in multiple components to be assembled and brazed together.

    For me personally, I only have the facilities to melt about 4kg of bronze at a time, so hollow casting would really only come into play when I want to do something larger. The bull was about my limit for a solid cast. For something like the reef sculpture you've made I'd have to make it hollow. Of course I'd have to change my mould making method for that, as I tend to do cut blocks rather than split moulds.
     
  10. Mantrid

    Mantrid Silver

    Not if the sculpture is hollow OR the sprue is thick and short and the pouring cup significantly bigger than the sculpture.

    A life size solid orangutang? That would be extremely heavy and be a huge cost in bronze. I cant see any logic in that. Bronzes that size are always hollow. Why does he want to make it solid?
    Regarding people wanting heavy bronzes. I think that may be the case for very small pieces but once they are bigger than your fist then the weight becomes a problem. My reef sculpture is about 6kg from what I remember and its hollow. People dont want things that heavy on their furniture and shelve especially if its glass, and they become a problem moving them around and putting them up on high shelves.
     
  11. Mantrid

    Mantrid Silver

    Another thing to mention put simply is that as something gets bigger the volume and therefore the weight increases much more than say the dimensions. What that means is that for small pieces like yours the weight difference between a hollow and small piece is not so noticable. However for a life size piece the weight difference between a solid and hollow piece is huge.
     
  12. Smoking Shoe

    Smoking Shoe Silver

    Lots of guessing on my part folows:

    Metal, and most everything else, cools at a rate dependent on the surface area to the mass. The center of a sphere being the works case for delta between the core temperature and the surface.
    The area between the head and the sprue is the point where this ratio is the least - it cools faster and therefore solid first blocking the flow of metal that could be used to fill the shrinkage of the still molten areas in the head from the still molten riser.

    The suggestion to change the alloy may help. Alloys have a property that there is a range between the liquid state and the solid where they get, to use a very scientific term, Slushy. They are neither solid or liquid in a range of temperatures known as solidus and liquidus. Small changes in the alloy can make a big difference in this range. Bicycle builders use this property to make nice neat fillet joints on their brazed frames. Once that little disk of metal between the sprue and the head is fully solid then as the metal cools in the head either the skin will suck in or there will be a void formed. More mass directly above the head with less taper on the sprue might help enough.

    Long story short - the temperature difference between that little disk area and the head crown needs to remain small enough long enough for all of the stresses to get resolved by transfer of metal from the sprue. I'm also - guessing - that if you section the bird you will find voids in the green area. No need to saw it up if you can measure the volume and weight accurately enough to calculate the density of the casting.

    freeze.jpg
     

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