Using casting sand as an sculpting medium

Discussion in 'Sand Casting' started by Tana García, Jan 30, 2023.

  1. Tana García

    Tana García Copper

    Hi there!

    I'm Tana, a designer based in the Netherlands. I would like to create sculptures made of discarded sand and I realized that the binders used for sand casting could help me a lot. I need to craft 1*1*2 m blocks and then carve them out, and I would like to use a binder as sustainable as possible (so epoxy or synthetic glues are out of the question). Because the blocks are that big, I can't bake them. I also tried binders that air dry, like starch paste for instance, but the core never dries and ends up growing mold.

    I've been using sodium silicate and propylene carbonate as recommended in one of the forum's threads. It gets bound, but I never have enough time to mix it properly; it starts to crumb up and then I can never get a "block" (I'm doing ~500g tests). I just get some lose hard sand lumps. I'm following the method presented here, by user @Melterskelter :

    All the componets are used at room temperature. Has anyone experienced something similar/has any suggestions for me?

    Thanks a lot, I appreciate the information posted in this forum very much!

    Greetings from Amsterdam
    Last edited: Jan 30, 2023
  2. Foundry Rat

    Foundry Rat Silver

    The size of the part makes sodium silicate/ CO2 mold difficult. Can be done, the trick is to get the CO2 through the entire mold. Plus you really need new clean sand, no clay. Clay gums it up.
    There is resin based binder that uses an acid to make the binder and sand hard. Need new sand for that also, no clay. The batches you need to make are large, I think you would need a barrow muller
    to mix it fast enough. Maybe it could be mixed in a wheel barrow with a garden hoe. That process is a one time use with the sand. Can be recycled but is very expensive to do so.
    Without seeing the part to make, I am inclined to think this should be lost foam.

    There are some really good lost foam people here, maybe help can be found with them
  3. HT1

    HT1 Gold Banner Member

    I have seen similar quiet literally sand castle sculptures made from green sand used in Malls as holiday pieces, they of course are temporary, and will after a month or so begin to crumble, I dont know any foundry sand that would be permanent even PUNB (polyurethane No bake) which is the most rigid sand I have ever used would crumble eventually (heat and Humidity is it's issues )

    V/r HT1
  4. Tana García

    Tana García Copper

    Hi HT1, Foundry Rat,

    Thanks for the info! The sculpture will be carved out of the block, and the concept is similar to a sandcastle yes.

    Regarding sodium silicate, I found out mine is the "weaker" kind. It's very hard to find the 2.4-2.6 SiO/NaO aroud here. Is there any kind of way to chang the ratio? Like just buying NaO and adding it? I don't know if anyone has tried this.

    This piece will be in a gallery in a controlled environment, and I actually wanted it to crumble in the end (as sandcastles do), so I'll test the durability as well!
  5. I found some recipes on youtube for making sodium silicate from silica gel kitty litter and sodium hydroxide. If you keep adding silica gel until no more will dissolve in solution you will end up with a suitable sodium silicate with a pretty high S.G. I still have some weathered chunks from over a year ago in the backyard after exposure to tropical weather. It also seems less prone to absorbing moisture, probably because all of the sodium hydroxide is reacted by excess silica gel. I was adding 8% to my sand by weight and it had similar properties to the phenolic resin bound sand in terms of carving.
    Tana García likes this.
  6. Tana García

    Tana García Copper

    Hi Mark, that sounds very interesting. How did you cure the sodium silicate? Did you just add 8% to your sand and let it dry out? Or CO2? Phenolic could also be an option, but it's quite toxic for the purpose I need.
  7. It was cured with CO2 gas, for your purposes you could bury a gas hose in the centre of the sand and ensure the center gets gas, it will get quite warm from the curing reaction. The cured moulds I made were stored out of the weather and sunlight but exposed to outside humidity and stayed quite dry for weeks at a time before use. In fact the water content of the sodium silicate dried out over time so the sand moulds became lighter in colour.

    The mould in the photo is about 25cm square and 60mm thick. The small darker dots are the 2.5mm gas holes in the sand pattern where the gas enters the sand, so small bumps of sand come out of the gas holes. This is the closest I've got to the phenolic urethane binder I have used elsewhere but it has a very long shelf life when stored properly and is relatively cheap too, both problems for PUNB with it's six month shelf life and expensive cost. It almost feels like concrete when handling with a higher strength than PUNB resin with the 8% sodium silicate content. The bottom photo was an earlier mould with uneven CO2 gassing before I modified the pattern to have multiple gas holes so the curing was uneven. There were also draft problems with the pattern too that caused sand to break.

    For a large block like you're making it may be a good idea to leave it in an envelope to retain the gas and let it sit for a while (days) to cure evenly before opening the mould.

    sodium silicate sand.jpg

    sodium silicate poorly gassed.jpg
    Last edited: Jan 31, 2023
    Tana García likes this.
  8. Foundry Rat

    Foundry Rat Silver

    Hmm....I wonder if this would be a job for molasses binder? Karo Syrup? Clear to keep the sand white...
  9. rocco

    rocco Silver

    How hot would you need to get a molasses binder to set up? And how difficult would it be to build a temporary oven large enough to bake a block the size the OP is looking to make?
  10. Tana García

    Tana García Copper

    That's why I discarded the molasses or the oil bound sand. Yesterday I attempted a new method of saturation since, as I mentioned, when I mixed the silicate/propylene carbonate with the sand it immedietaly started to clump up. I decided to dilute the components in water and saturate the sand with the mix, allowing the waterglass to react with a bigger time margin. After 15' it started to gel, so I have hopes for this! Maybe it's useful for casters that had the same issue with their chemical binding.
  11. Foundry Rat

    Foundry Rat Silver

    If your sand was clumping up I would say you were using far more binding agent than you should. Sodium Silicate . I would never dilute it. never saturate.

    Tana, I was having a hard time understanding what you are trying to do, I thought you were trying to cast something. I went back and reread, it is all clear in the first sentence...
    The spent sand, what kind is it? Is it spent green sand? Is the sand black?
    Tana García likes this.
  12. Tana García

    Tana García Copper

    Hi Foundry Rat, for the moment I'm just using regular construction sand (1mm) that I'm drying out prior to doing the experiments, so I can control the moisture levels.

    I'm using 5-10% binding agent at the moment.

    From your first suggestion, I did some very rudimentary co2 gassing (made a little tennis ball and cured it over some baking soda and vinegar) and I was very impressed. The outcome is the perfect texture I need. Since I'm going to sculpt the block, I'm not sure I can use the holes, or if I use them there cannot be a lot of them. I'm also considering doing layers: doing, let's say, a 5cm layer, gassing it up, then another 5cm later, gassing... I hope I'm making myself clear. What would you say is the maximum thickness I could use with this method?

    Best regards, thanks again for the help!
  13. Melterskelter

    Melterskelter Gold Banner Member

    Tana, I think I would try using the sodium silicate and propylene carbonate and experiment with two factors:

    1) Lower the temperature of the sand and environment. Being winter, that may be more feasible now than in summer. If your ingredients were in the 40 F temp range, I suspect the reaction would be much slower.

    2) Reduce the ratio of the catalyst to silicate ratio by maybe a factor of 10 like 1:100 rather than 1:10 and see if the reaction slows down enough. I do not know how you are mixing the sand and sodium silicate, but a portable cement mixer seems appropriate. For batches of a few pounds I use a powerful hand-held drill motor and spiral paint mixing paddle like this

    I think a batch of ten pounds in a five gallon bucket could be done this way. But bigger batches might be a problem.

    CO2 curing of a large block could be done if thin holes were drilled into the sand and a tube used to insufflate CO2. Beware of over-exposing the silicate to CO2 as this causes crumbly results. I use just 5 seconds bursts of CO2 into the holes spaced a few cm in my few-pound cores.

    I really do think cold temps are your easiest and best bet. Next, I'd try the dilute catalyst. I'd use about 5 pounds of silicate in 100 pounds of sand. In warm air I try to get my sand and silicate mixed in just 2 or 3 minutes.

    Tobho Mott and Tana García like this.
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The problem with gassing large SS molds is getting the CO2 to diffuse deep into the molds. This may or may not be practical for you, but I used to do a lot of vacuum bagging to form composites. I have placed fairly large SS molds in a bag, connected the bag to vacuum source, evacuated it, and then allow it to reinflate to atmospheric pressure with CO2. I use polypropylene film and have a heat sealer to make inexpensive bags than can be resused/recycled. Removing the air means the CO2 will penetrate the mold. Sometimes several cycles are necessary or partial evacuation to limit curing and CO2 exposure. Perhaps a bit cumbersome but it can allow use of undiluted SS and the ability to vary and achieve strong bonds with very long work times. You still need to be mindful of reaction heat when curing.

    Tana García likes this.
  15. Tana García

    Tana García Copper

    Hi Denis,

    First of all I'd like to thank you for your posts about sodium silicate, they led me to this binder in the first place!

    I'm working in an environment that's about ~17C (62.6F), and the sand is cooled completely, but still the sand clumps up as soon as it comes in contact with the SS/PC mix. I think the catalyst to silicate ratio might be the answer, I will try it out today. Another question, what order do you mix them in? I'm first mixing the SS/PC and then adding it to the sand, is there maybe a better way to do it? And is water added/needed at all?

    Now I want to do 15kg (33pounds) blocks to start carving, and I will use a concrete mixer with a paddle very similar to yours.
  16. Tana García

    Tana García Copper

    Hi Kelly,

    That is actually a great idea. I have worked with composites and vacuum bags as well and sounds like it could work, although I don't know at what scale. I read about using a regulator for the CO2 in other posts, is there an estimate of how much CO2 is needed per liter or so? What do you think of the layers idea?

    Also, when you guys say that the mold gets very hot when curing, what temperature are we talking about?

    Thanks <3
  17. Melterskelter

    Melterskelter Gold Banner Member


    First I pre-mix 1000gm Sod Silicate RU +150gm tap water +150gm table sugar (you don't need the sugar as it is added for weakening the core post casting and making its removal easier. I do not thinkit affects pre-casting strength. I actually keep a couple of liters of this premix on the shelf.)

    Then I weigh out dry clean 100 mesh sand 1000gm at room temp (70F) and put
    it in a suitable bucket----maybe a gallon plastic bucket for 1 kg. In the winter, if my shop is cold, I warm the cold sand to 70ish using a hand-held propane torch and a metal bucket.

    Then I weigh with an accurate balance beam scale 60 gms of the above SS mix. To that mix I add 6gm of Prop Carbonate (obtained from Clay Planet in the US) and stir the PC into the SS.

    Then I dump the SS/PC into the sand and mix it vigorously tipping, rotating, and bouncing the bucket and scraping down the sides to ensure uniform mixing. Mixing that amount of sand probably takes a minute. It feels barely moist and is not clumpyat all.

    Then I dump it into the waxed mold (I use Johnson's wax or Minwax and ususually wax the mold two or three times lightly buffing it) and and lightly tamp it into place. You might also lightly pre-talc the mold to ease removal. SS can react (bind to ) with some paints so I like to use epoxy paint if practical or true lacquer like Rustoleum Lacquer as a second choice. Metal and plastics should not react.

    I do not know how exothermic the SS/PC is. It must be somewhat, but I have never noticed in the few pound batches I use. I do think that is amvery real concern in the massive batches you anticipate.

    I like Kelley's suggested bagging idea. It won't take a lot of CO2 to kick off your SS (I think I'd skip the PC if CO2 fixing the SS.) I use a small CO2 bottle about 18" high and it has lasted a long long time on a single fill though I do not use it too often---usually if I am in a hurry.

    I do think cooling the sand may be helpful for large batches. Madrid's temps recently were 50F daytime and freezing at night. So,you could take out, as an example, a pound of sand and leave it out over night getting its temp down to 40F. Mix that with a pound of 60F sand and you would have 50F sand that would kick off much much more slowly than 70F sand. Similarly putting 1 pound in a freezer cooling it to 0F would allow you to cool a few pounds of warm sand to 50. Myself, I'd probably be somehow using buckets and a handtruck to eventually move hundreds of pounds outside spreading it out on a tarp to cool and bringing it in the next day to mix up cold. But that might not be practical in your circumstances.

    One other approach could be to mix up and cast 1 cu foot blocks of sand and then using mixed fresh sand/SS mortar the blocks together. I have never tried this. It MIGHT work. It might not look good for sculpting though.

    Interesting problem.

    Last edited: Feb 2, 2023
    Tana García likes this.
  18. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    You can use a regulator for CO2 but it isn't necessary. All you need to do is have a valve that allows CO2 to reinflate the bagged/evacuated mold. When the bag relaxes from the mold it has reinflated with CO2 to atmospherice pressure. If you re-seal the bag, sometimes it will once again go into slight vacuum as CO2 is reacted by SS. You should use a gauge to measure level of vacuum achieved. You don't need to pull high vacuum. In fact, you may find you get best results at a fraction of an atmosphere because this has the same affect as diluting the CO2, but for me, I was able to easily achieve .1-.2 ATM vacuum and did so twice and my molds were solid. You can also fill a plastic bag with CO2 and expose the evacuated mold to the bag full of CO2 as a crude means to measure CO2 added.

    If you have vacuum bagged before, you probably know it can be difficult to get good vacuum because the bag tends to clog the hole from your vacuum source. I made a distribution header which was simply a piece of pvc plastic pipe with holes drilled the length of the tube and used a table saw to cut a shallow slot connecting the holes the length of the tube. You might be able to work this into your mold.

    I recommend you experiment and try it on a small scale to see how you like the results. It's hard to say how much gas you will need because it is so dependent on SS concentration, temps, etc. You just need to experiment but very weak SS concentrations may never harden sufficiently.

    As far as layers, trying to get CO2 to penetrate the surface without penetrating the surface with a gassing needle is futile, and if you are going to bag it, may as well do that in one shot. If you think you may do a lot of this sculpting, controlling the SS concentration and percent by weight addition to the sand, along with mixing the sand and gassing (if you go that route) will be important to getting repeatable results.

    Hard to say. It will depend upon SS and gas concentrations but mine were never more than slightly warm to the touch. Sand is fairly insulating so the same thing that creates higher core temperatures limits how fast heat can be conducted outward. It's probably a non-issue unless you have very high SS concentrations. Foundrymen usually dont encounter this because they keep SS concentrations relatively low so the cores don't become too hard and difficult to shake out and remove after casting. SS cores exposed to high heat can become rock hard, thuse Denis' comment about adding sugar. I doubt you'll have that problem. Longevity of the culpture is also difficult to predict.

  19. Tana García

    Tana García Copper

    I made a little block (500g in a mixing cup) with way less catalyst (1:100), no water added, and it worked wonders! I'm scaling it up this afternoon and check if it works on a bigger block. No water added, and in a few hours it was very solid. I think we got it! I'm still curious about the CO2 though!
  20. Tana García

    Tana García Copper

    Hi guys!
    Just an update, I made a bigger block (15kg, 35*35*12cm) and it worked very nicely, it's even harder than what I expected. I used 10% SS, although it's not RU, it has a different percentage of SiO/NaO (3.3), but for the moment it works! I'll use 7.5% next time, see how it performs. I got a canister of CO2 regardless, so I will try that out as well.
    Melterskelter likes this.

Share This Page