Help needed- bronze sword/blade casting

Discussion in 'Sand Casting' started by Conor, Sep 29, 2023.

  1. Conor

    Conor Lead

    Hi all,
    after having some success casting some smaller replica bronze artefacts such as axes and a handful of short swords of around 30cm (only a few months new to this hobby), I have run into a couple of problems when attempting to cast regular sized bronze swords. Basically, I just can't seem to get proper edges on them. I have had success with two or so blades, however the rest have all been defective. I can't put my finger on what exactly I am doing wrong: pour rate is slow and even, the bronze i'm using is nice quality, and I can't see any problem with the mould making technique: basically identical to how Will Lord shows in his video here

    Sand: Petrobond. Vertical pour.
    Have tried both angling the mould slightly and while it does provide a slightly better finish, it does not help the problem with losing some of the edges.

    My patterns are made of wood and the thinnest part of the edges are around 1mm- I deliberately made the patterns thicker on the edges to try and avoid the very problem I am having! However, most people I see casting swords use an actual sword as their pattern which would obviously have been sharpened and have very thin edges. So I'm not sure if the pattern is my problem or not.

    Here is the sort of problem I'm having: As I say, the very odd time I do get a nice cast but really feel like I should be having a higher rate of success than currently.


    One of my (very bad) castings with the pattern on the right:


    If anyone has any experience in this area, I'd love to hear from you.
  2. Judging by your photos, the bronze needs to be hotter, I wouldn't pour something that length vertically as the hydraulic head will try to extrude the liquid bronze into the petrobond to some extent. You may get away with it at lower bronze temperatures to some extent but it will be an issue. The only bronze swords I've helped pour were quite thick so we were able to pour horizontally from the handle end and there was minimal shrink in the middle of the sprue. For something thinner you'd likely need runners or cast a much thicker blade and thin with stock removal to get to final shape. I've noticed bronze casting in general is sensitive to metal temperature: just moving from an A20 to a smaller A16 crucible size meant running at a higher temperature and having to pour fast to minimize cooling. The bronze swords pictured below are about 20mm thick/ 0.8" at their thickest point and can be barely lifted let alone swung due to their weight. The owner was going to spend a lot of time grinding down to size and they were going to be some kind of ritual temple sword/ornament.




    Conor likes this.
  3. HT1

    HT1 Gold Banner Member

    your metal is terrible! you can see all sorts of dross (Bi metal) related issues probably from the very long fall of the metal,
    the only way to overcome that in a vertical pour is to get the metal in extremely quickly so all the dross floats to the top into the Gating system (basically a riser)
    I'm just going to assume you are starting with good clean metal properly skimmed, if that is not true, fix that first,

    I assume you know Bronze swords are forged, work hardening in the process, so your thick edge is normal, after casting the blade would be hammered to a finished edge COLD, which will work harden it ,

    If you cannot get a better finish pouring vertically, you will have to pour horizontally, for the very best finish you would want the part of the sword farthest away from where the metal enters raised 15 degrees, this will allow the blade to fill very gently, not creating any bimetalfilms , if the metal flows down hill there will be turbulence, creating dross,

    Actually the very best way to fill this is pretty much a no go for the hobbiest you would need a vertical pour but a sprue going all the way to the bottom, so it fills up gradually from bottom to top, yes your metal would need to be at the top end of it's pouring range , this very long sprue would have to be quite small at the choke point .

    V/r HT1

    P.S. from your pictures, it looks like you are pouring dross into your casting , are you skimming the metal before pouring?
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  4. metallab

    metallab Silver

    This indeed appears to be a too cold pour. As HT1 mentions, vertical castings that long tend to cause much hydraulic pressure on the end of the sword. Bronze is almost 8 times as heavy as water, so the hydraulic pressure is 8 times as much as a water column that length. A good idea is what he says: make a long sprue towards the very end and let it fill the mold upward. That also mitigates the chance of slag / dross inclusions.
    Conor likes this.
  5. Conor

    Conor Lead

    Thanks- so just leave the metal in the furnace that bit longer? I try to add a small little sprue at the tip of every sword for this reason, but maybe I need to make it longer. Thanks.
  6. Conor

    Conor Lead

    Thanks for the reply. I generally flux with a pinch of borax first- and once I remove the crucible before casting I skim with a spoon. But generally there seems to be very little dross on the top- indeed I remember this casting hadnt really any so I didn't bother skimming.
    So you think maybe a quicker pour may help?
  7. HT1

    HT1 Gold Banner Member

    If I implied that it was not my intention,
    we are getting seriously deep into the weeds

    Go read this entire thread:

    digest it slowly, ITS VERY ADVANCED!!!

    in simple terms you need to get the metal into the mold as quickly as possible without agitating it . those two thing fight each other, they are close to antithetic, but not quite

    here is an example, not a great one, but an example , you have a casting getting filled from the bottom, now picture the casting is 3 foot long but quite thin, so we raise the right hand end up 15 degrees (Mold manipulation) so the entire casting fills without creating any Bifilms , as soon as you see metal in the riser at the bitter, upper end of the casting, pour metal into it , this will add lots of heat to the coldest metal (BTW if you are going to do this insert that riser at 15degrees so it is straight up and down as poured)

    Now that said the gating system, particularly the Choke becomes critical if it is too small metal will spray into the mold, picking up tons of dross, if it is too large, you will not be able to pour fast enough to keep the gating system full and it will "gulp" air also creating tons of dross, this gating system has to be quite small at least at the choke.

    I believe in the thread above I mentioned that as a general rule hobbiests are using far too large of sprues ( the base of the sprue is often your choke) i cannot see many hobbyists needing anything larger then a 1/2 inch sprue.

    Horn gates have a tendency to cause turbulance because of their shape , but mostly because the choke end is too large.

    V/r HT1

    P.S. if I did the sword as I recommended above, i would gate onto the Point of the sword not the hilt, the riser would be attached to the hilt, additionally unlike our picture the sprue could be beside the ingate, rather then downhill of it to save space in the flasks , and that sprue should be inserted at the 15 degree angle like the riser... told you it was deep

    P.P.S it could also be knife gated mold manipulated uphill, but that would almost double the amount of metal that had to be poured, the Knife gate would be almost as big as the sword
    Last edited: Sep 30, 2023
    Conor likes this.
  8. Conor

    Conor Lead

    Thanks for the advice HT1 and will look into that thread. However I have no wish to get into seriously advanced stuff- I mean I see tons of guys on Youtube with the same system as me achieving better results than I am and just wondering what link in the chain I may be getting wrong
  9. HT1

    HT1 Gold Banner Member

    first and foremost it looks to me like your metal is dirty from the start
    Im going to say this, and someone will hold it against me; do you scrape the interior of your crucible just before pouring to dislodge any dross attached to the side of the crucible? if not it maybe coming loose as you pour and going right into your casting.

    Let me simplify, once everything is liquid, that is when you flux , and scrape the sides of the crucible, the flux will take dross back to metal as much as possible,
    if you are doing some sort of cover(crushed glass ) I would put that in with the initial charge , if you are doing charcoal powder that needs to go in as soon as you have some liquid, and you may have to do more before pouring (depends on how well your furnace heats). take the metal to the pouring point then degass, in your case with a tin bronze you will need Phos Cu, i believe its 1% of 15%, for gods sake make sure the Phos Copper is Dry, I like to wrap it up in an aluminum foil packet, place that in your plunger, and preheat it all, Just before pouring plunge that to the bottom, hold til it stops boiling , remove the plunger, touch up the skimming if anything floated up, pour

    V/r HT1
  10. Here's another vote for scraping the inside of the crucible while molten to bring any crud to the surface. Like HT's post above, I'm scraping the crucible inside and skimming out the crud, then fluxing with a bronze flux powder, add a commercial granular coagulant which melts into a sticky lump that can be removed. These are done while the bronze crucible is in the shut down furnace which is then started up to let the coagulant work and keep things hot.

    Last of all is a short piece of copper tube with a small amount of 15% phos-copper shot and the ends crimped off, it's wedged into the split end of a steel bar and plunged into the bronze to melt. If you are pouring with a helper, you can float a small block of hardwood on top of the bronze to keep the air off the bronze and your helper can use a skimmer to keep the wood from falling out as you pour. A helper can push any dross/crud away from the crucible spout as you pour the bronze.
  11. HT1

    HT1 Gold Banner Member

    Your getting Very professional, all techniques I know about but very out of the hobbyist realm.

    V/r HT1

    On a side note: why is the metal melting industry so Damn secretive?
    commercial granular coagulant? did Vesuvius not name it... Probably not! a lot of their products aren't named. go into a Supplier, and all the bags are identical, only the pallets marked , and this along with the impossibility of getting small amounts of product create a weird Monopoly on Knowledge in the metal melting industry,

    P.P.S But everyone feels free to use the word Foundry for WTF they feel like.
  12. I had a sample of the Foseco coagulant and it's some mineral rock similar to the unexpanded perlite: swells up and melts in a propane torch flame. I'll find the post I had about experiments with it, the local perlite miner/processor said they sold it by by the tonne to the local iron foundry.

    So the local mined unexpanded perlite is on the left and the right is the commercial Foseco coagulant after both had a hit with the blowtorch. I can even lower the melt temerature further by adding washing soda/sodium carbonate to flux it into glass.

    Last edited: Oct 1, 2023
  13. HT1

    HT1 Gold Banner Member

    same as my experience with Cast Fe , but Im sort of "why" on it's use with Brass/bronze unless your metal is nasty to start with simply skimming off the dross is easy enough, I work entirely with scrap so I see plenty of dross, but i do stay away from real problems like turnings where there is more dross then metal, and lots of really bad contaminates like sulfer,

    V/r HT1
  14. We're running the bronze fairly hot so it acts as a bit of cover towards the end and speeds up the final skimming by having a semi solid lump with all the crud in it. The A16 crucible needs to noticeably hotter than the A25 for some particular moulds with their thin sections.
  15. Conor

    Conor Lead

    Agreed. Dont get me wrong guys, i appreciate the time you are taking to try and help me out, but im just a guy in my backyard casting relatively simple bronze age tools. Also i think that what some of you are referring to as dross in my casting is actually just black petrobond still stuck to the casting. I didnt really clean it up before taking the photo. Yes the surface is rough but think this is more down to the sand not being as smooth as it could gace been before pressing the swird into the mould (despite always being as thorough with this as possible). A lot of that black crud is now gone after a simple going over with a wire brush.

    I know the front runner for a solution to my problem seems to be to make the pattern thicker. However what i cant understand is how ive seen many people use an actual finished bronze sword as their pattern, which will obviously have thin edges, been work hardened etc.
  16. HT1

    HT1 Gold Banner Member

    I would suggest if you have a team, and it sounds like you do, take some immersion temps right up to the point of pouring, if you have an extra body he can document temps going into the mold, this will give your some really good data you can use for planning, I have never noticed significant cooling from first to last mold, I can do 7 with A 20 I used to get 6 Mostly with an A 16, So you maybe pouring too cold in general, it could also be your head pressure (cope height) is a little short, this can be overcome, by an aggressive pour and overheating (what Im saying is you maybe a little over optimized and on the bleeding edge of failure at some area, maybe temp, maybe something else, pin that down to get rid of failures)

    V/r HT1

    P.S. I assume you are doing everything in the furnace if you are pulling the crucible and degassing, skimming ect, outside that is an issue for heat loss
    Last edited: Oct 1, 2023
  17. metallab

    metallab Silver

    I found a few 'swords' which I replicated last year from an African original made from hardwood which I cast in (left to right on picture)

    wood original used as pattern - cast iron - copper - bronze - bronze - brass

    Length: about 25cm (10"), wall thickness max 5mm (3/16").

    I made a horizontal mold and did not use a riser and the gate was in the center of the sword with a short runner of about 3cm from the sprue. I used greensand.
    All casting succeeded.
    So tilted horizontal might work.

    And about 'dross': Dross are solid contaminations mostly appearing with lower melting point metals like aluminum and sometimes brass (zinc oxide). At higher temps a liquid (called slag) is formed. Maybe the petrobond sand TO (topic opener) uses forms dross as well, after pouring, because it decomposes to bare silica sand with black carbon. Maybe green sand as I use is better.

    Attached Files:

  18. Yes all the skimming, fluxing etc is done in the furnace with it off and then given a few minutes, 5-10 mins to get the heat back into it. I'm helping my friend pour this bronze casting. We'd been using the A25 for so long that going to the A16 caused a few failures, only just barely not filling the thin rib section. So there was a need to adapt to the smaller crucible and run a bit hotter and not waste time once the crucible was out of the furnace.

    I'd expect Conor would be using a relatively small crucible and need to minimize heat loss for such thin sections, maybe have a pouring spout large enough to be an easy target to hit and get the metal in fast.

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