BristolPotter Posted April 16, 2023 Report Share Posted April 16, 2023 Hi everyone, I'm new! I currently make a very small range of slipcast items from my own moulds (created using a resin 3d printer, silicone and then plaster) and I would like to be able to use stoneware slip for its plasticity and cost-effectiveness. At present I mix my own porcelain slip that has a S.G. of 1.8, using a dedicated slip mixer, not a hand drill or anything. Every time I try to mix stoneware casting slip in the same way, I can't seem to get it past 1.6 S.G. It ends up over deflocculated if I try. I should mention that I'm in the UK and I can't get Darvan, I use sodium silicate as my deflocculant, with a little soda ash, and then Dispex for fine-tuning viscosity. It all works perfectly well for the porcelain, but the same process does not work for stoneware. Does anyone out there cast in stoneware and make their own slip? I wonder if I'm expecting too high a S.G. from stoneware, or if there's some magic trick that I'm missing to make it behave as well as my porcelain? Thanks for any suggestions! Quote Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted April 16, 2023 Report Share Posted April 16, 2023 (edited) Are you buying in the clay? I wouldn't think there's much price difference if you are making it up from raw materials? I was actually doing some tests yesterday to try and get 1.8sg, it has been pretty easy to get to 1.7sg with: 1000g Dry Material : 430g Water : 3g Sodium Silicate (140s) : 0.1g Soda Ash Yesterday I tried 1000g Dry Material : 400g Water : 7g Sodium Silicate (140s) : 0.1g Soda Ash (It was 1.75sg after initial mixing but 1.8sg this morning which is a little odd. Probably a measuring error on my part) 1000g Dry Material : 375 Water : 5g Sodium Silicate (140s): 1g Soda Ash (1.8sg after initial mixing and still 1.8sg this morning) They are slightly more viscous than I would like and I have read that going over 0.3% Sodium Silicate is not great for mold life but they cast fine this morning and poured out easily after 20min in the mold. My clay recipe is Hymod AT Ball Clay - 60 Nepheline Syenite - 30 Petalite - 5 Alkaline Frit - 5 Not exactly stoneware as it is fired to cone04. I ran out of Neph otherwise I would do more tests. Thought about using soda feldspar instead as it should act the same with my deflocculation tests? Edited April 16, 2023 by High Bridge Pottery BristolPotter 1 Quote Link to comment Share on other sites More sharing options...
BristolPotter Posted May 9, 2023 Author Report Share Posted May 9, 2023 Yes, I'm buying in the powdered clay from Potclays, it's their 1145. I have been using way more sodium silicate (based on the recipe for casting slip from Potclays themselves), so I suspect that earthenware might behave quite different to stoneware Quote Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted May 9, 2023 Report Share Posted May 9, 2023 Yea they are saying to use 3.5g for every 1000g of dry material but I haven't found that to get anywhere near fluid enough and I am using double that at 7g for every 1000g of dry material. Quote Link to comment Share on other sites More sharing options...
Jeff Longtin Posted May 9, 2023 Report Share Posted May 9, 2023 Hello Bristol Potter. Yes, I cast porcelain and stoneware. (Porcelain in my studio and stoneware at the pottery shop.) I don't mix stoneware from scratch but I do mix it from premix powder. (Laguna B-Mix.) Your comment about SG left me puzzled so I held off replying initially. The premix has all FOUR deflocculants in it. Darvan, sodium silicate, soda ash, and barium carbonate. Not a combination I've ever used but it does work. It's been a bit of trial and error but I finally got it down so we have a SG of 1.75. The flow, thru a Ford cup, is about 40 seconds. Lagunaclay.com has several recipes you might look at. (Go to More than go to Clay Resources.) There you will find several variants using the above combination. It sounds like you have your own clay body but at least you might try some of the deflocculant combinations they suggest? Good Luck! Quote Link to comment Share on other sites More sharing options...
PeterH Posted May 9, 2023 Report Share Posted May 9, 2023 On 4/16/2023 at 8:34 AM, BristolPotter said: Every time I try to mix stoneware casting slip in the same way, I can't seem to get it past 1.6 S.G. It ends up over deflocculated if I try. >Yes, I'm buying in the powdered clay from Potclays, it's their 1145. I have been using way more sodium silicate (based on the recipe for casting slip from Potclays themselves), so I suspect earthenware might behave quite different to stoneware I'm a little confused. Looking at Potclay's site they seem to give a "starting point range" rather than an actual recipe. Can you confirm this is what you are trying to use?https://www.potclays.co.uk/clays-slips-grogs/clays-slips-grogs-powdered-clays/white-st-and-39;ware-powderhttps://www.potclays.co.uk/recipe-casting-slip-powdered-clay As you don't have a definitive recipe I assume that you are doing small-scale experiments until you work one out. I assume that you started by exploring the range of recipes in Potclays recipe-range. Perhaps by a some variation on "keep slowly adding deflocculant until you reach the sweet spot" (or more precisely until you've overshot it a bit, but can reconstruct the recipe at the sweet spot). A rather formal version of the process is given in Determining Dispersant Additions for Casting Slipshttps://static1.squarespace.com/static/527ac372e4b0d4e47bb0e554/t/527fd7f1e4b0c046bfa9b90d/1384110065234/Dispersant+Addition+Procedures.pdf My guestimate based on Brongniart's formula suggest that these slips would have a SG of about 1.75 to 1.78 (see PS). So if the slip at the sweet-spot had been acceptable it would probably have met your needs. How bad was it? If the slip at the sweet-spot wasn't acceptable it might be worthwhile asking Potclays how you should proceed, as it makes their advice look potentially suspect. >Yea they are saying to use 3.5g for every 1000g of dry material but I haven't found that to get anywhere near fluid enough and I am using double that at 7g for every 1000g of dry material. But it's Potclays recipe advice for their clay. If their advice has any validity surely this should be resulting in significant overflocculation. Either something is badly amiss or Potclays recipe is totally misconceived. BTW can you confirm that your sodium silicate is 120tw and your soda ash is fresh. PS I note that Potclays recommend 25kg clay in 10l of water. So I'll try to work out its SG. Based on http://www.potteryatoldtoolijooaschool.com/brongniarts_formula_made_easy.pdf SG of clay is 2.5 to 2.6 And making Brongniart's assumption that the solids don't dissolve, but simply occupy their volume in the final slip ... Weight of slip = weight-of-clay + weight-of-water = 25kg + 10kg = 35kg Volume of slip = volume of clay + volume-of-water ... between 10 + 25/2.6 and 10 + 25/2.5 litres = 19.6 to 20 So SG of Potclay's slip is weight/volume = 1.75 to 1.78 Quote Link to comment Share on other sites More sharing options...
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