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faq

How to mix dry pottery slip

James Pagni

Mixing slip should be easy but when you don't know what you are doing, it is nearly impossible to get it right. So when you buy dry slip from us, you are going to get a mixing recipe. It is pretty easy to follow the instructions but materials can still play games with you. You would be surprised to see how much cold water versus hot water makes a difference, or filtered tap water or not filtered, and even the humidity in the air. The smallest changes can be effecting results but only by so much, it shouldn't stop you from a successful mix. Let us briefly go over how to mix slip and check that it is correctly mixed.

So when starting out you want to weigh how much dry material you are going to use. In a separate bucket we will be adding water. With adding water, a good average starting point is 40% water. So if we have 10Lbs of dry slip, we are going to want to add 4Lbs of water (64 fluid ounces). When we break that down into how many fluid ounces of water per pound of dry slip, it equals 6.4Fl oz.
Image result for water in a messuring cup
Once you get your water in the bucket, you will want to add darvan#7. Darvan is a deflocculant that basically separates all clay particles from sticking together so it will be liquid. If you just added water it would stay clumpy and thick. We can add .03% to .05% Darvan per each pound of dry that we will be adding to the water. The math on that would be: 16oz(1Lb) x .003 = .048oz of Darvan and that translates to 1.4mL of Darvan per pound. This is a good point to start from and you can always add more but if you add to much, then you need to add more clay and water to even it out.
Image result for Darvan#7
Now we have 64oz of water and 14mL of Darvan in our bucket. Let us start adding the dry slip mix (where a respirator and be in a well ventilated area). Once you add it all to the bucket you can start mixing it with a drill. Start mixing it and once it looks like all of the dry material has been mixed in, then you can call it good to rest. It is best to put lots of energy into mixing it for best results. We have a slip mixer that we mix the slip for several hours or until the slip gets nice and warm. You will also want to let it sit for a few hours, 8 hours is good. You want all of the dry materials to break down into the water before our next step.
Image result for mixing clay slip
Now that it is mixed and sat for some time we can sieve the slip. Use a 40 mesh sieve or greater to remove extra impurities from the mix. If you feel comfortable trusting me with the mix I gave you then go with it, but if you like double checking yourself or if issues arise then lets do this step. Get some type of volume measuring device like a measuring cup or the best is a plastic syringe. So weigh the measuring device to zero it out on the scale. Then add 30mL of slip and write down that weight. Then clean the slip out and weigh 30mL of water. Now we divide the slip weight by the water weight. So if the slip weighed 44 grams and the water weighed 25 grams, then we go 44 divided by 25 = 1.76 So you SPG is 1.76
Once you have the specific gravity around 1.75 to 1.8, then you are good to go. You might want to check the viscosity of the slurry as well. Now we don't do this because it is usually close enough from our mix. But viscosity will basically tell you how your fluid is flowing. If it is to thick but your specific gravity is perfect, then you might need a little more darvan to make the mix flow better (add less Darvan then to much to start, maybe .25mL for small batches. Start mixing the slip till you almost have a vortex going and then pour the Darvan into the vortex to see how much deeper the vortex goes from mixing it in. Once the vortex stops getting deeper then stop adding darvan, or if it is at a better consistency). You can use a viscosity measuring cup to help here. But for us, we have mixed it so many times that we know how it is supposed to flow off of our mixing blade. When you have used the same mixing recipe too, you will come out with the same results almost always.
 See the source image
So I think we went over what you need to know so you can start to mix your own dry slip. It is not to hard but slip can hold you up when your going in blind for the mix. If you have any questions you can comment below or email us for a faster response. Remember to check out our molds and slip for when you get started. We will have our slip available by the gallon mixed in the future. But we sell it dry in hopes that you will learn to do it on your own.
Image result for slip casting plaster molds

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