Clay Shrinkage - How To Use That Number

this video and my most recent blog post are about clay shrinkage so you probably know that play shrinks it's around 10 to 20 generally more close to 10 unless you're working with porcelain it's the percentage decrease in size from when it's fresh out the bag to generally when it's fired and they might give you a range of temperatures because the amount it shrinks depends on what it's valid to um the initial shrinkage as it dries is from water evaporating and then there's chemically bound water that's given off in the higher firings and then when it's taken to vitrification some of the silica melts and all of this means that the piece gets smaller it's important to know shrinkage percentage if you know if you have a defined size you want to throw to and you want to know how big you're going to throw your piece but the way you use the shrinkage percentage to work out what size to throw your piece it's quite a simple calculation but it isn't the one that I see most people using the way that most people do it is ever so slightly wrong and I will explain why that is I'm going to throw a couple of pieces and demonstrate what I how I use the shrinkage in my work start with I've got this grid something I designed and is produced by Hartley and Noble to go with their Russian doll bat system so it's an insert that goes in and gives you a grid these measurements are absolute measurements so one centimeter on here is one centimeter so I can set my laser guide which is what I use as the Mark I can set this to the measurement I want to throw to and it will give me a 3D point in space that I then if I can get hit that with the top corner of the piece the piece is the right side so the piece is the size that I'm aiming for it should shrink down to the Finish size I want now the thing is you have to know what size to set the laser to so I'm going to throw a large pretty slippy mug but all my large swirlies and impulse they're all the same size and the Finish size is 10 centimeters tall by nine and a half centimeters in diameter I have set this to 11.1 tool and 10.6 in diameter approximately um there's a little bit of wiggle room that can't be too precise with it but broadly speaking that's what I'm aiming for how do I get to those numbers well the way that most people seem to do it from what I've seen sounds right it makes sense so this clay shrinks ten percent and most people make the throne pieces 10 bigger so you throw to 110 of your finished size that's not quite right very close you can see why that's not right if you use a hypothetical clay that's got a much higher shrinkage so if you had a clay that had a 50 shrinkage then you would need to throw twice as big it's going to halve in size when you throw so you throw to double so say you wanted it to be 10 10 centimeters when it was fired you would throw it to 20. but if you were going using that system of adding the shrinkage to it you'd throw 50 bigger 150 15 centimeters when you fired it that'll be seven and a half centimeters if you pick 10 centimeters because that was the size of your lid you're going to be quite disappointed when your piece comes out seven and a half centimeters the reason it doesn't work is because the 10 represents the size from throne to the Finish piece so the finished piece is 10 smaller it is 90 percent of the throne piece in order to get from 90 to 100 you don't add ten percent or not 10 of 90 if you see what I mean what you do is you divide by ninety percent or essentially divide by 90 and then multiply by a hundred which dividing by ninety percent will achieve the same thing and what that means is you're actually working backwards from the 10 to get the size that the finished pieces and then you divide the Finish piece by that to get the throne size sounds a bit Messier and that I think is why most people don't do it that way intuitively it sounds sensible to just add the shrinkage percent to it and when it's 10 the difference isn't actually that big so if you've got 10 centimeter piece like I'm throwing here and you wanted to you wanted it to finish at 10 centimeters and you added 10 the way that other people do I'm saying is not the correct way you'd come to 11 centimeters as the throne says if you divide by 90 you get to 11.1 centimeters it is not a huge difference between the two now obviously that difference could be the difference between a lid fitting and not um and it gets the discrepancy gets bigger at higher shrinkages but um yeah it's not actually that big a deal I think that's why most people don't even notice that it's not 100 right but that is the correct way to do it so I think if clay supplies gave you 90 rather than 10 as the the number it would be intuitive to work it out the other way but because you're given 10 the easiest thing to do with the 10 is not to subtract it from 100 to get 90 but just add it but what you do actually need to do is do it that way so you take your clothes shrinkage that you're given whether it's 10 15 whatever um you subtract that from 100 to get the percentage of the final size and then you divide the size you want the finished piece to be by that percentage so 10 centimeters divided by 90 gives you 11.1 centimeters which is how big you have to throw a piece for it to shrink down and you can do that with any percentage so if you divide it by 85 or if you get obscure ones like you can go as precise as you want and it will work um a calculator should be able to understand that if you're using a more rudimentary calculator that doesn't have the ability to understand divide by percentage you can convert a percentage to a decimal just by putting naught point and then the two numbers so 0.85 will be 85 and doing that the same that will work so you could do 10 divided by 0.85 would give you um the scale up for a clay that was 15 shrinkage now all of that's well and good it's not a calculation that I ever actually do and the reason for that is so hard to do this and talk got a spreadsheet with all of the final sizes and I've got tabs for each of my clay that give you the throne size in that clay so a different shrinkage for each clay and then all this up all the pieces should match regardless of what size what Clay they're throwing um some people on Instagram asked if I would share it so I have added to the blog post and I will link um below a shared version of the spreadsheet it does up to three Clays although you could modify it to add more if you wanted but three clays with different shrinkages and as many pieces as you want um and it will automatically convert so you don't need to understand the the way that you do the calculation um it will do it all for you so that's linked below as well um and yeah that's pretty much it it's not a huge deal but if you're doing it wrong this should hopefully help you do it right um the camera's running I might as well throw another I won't talk at the same time

Channel: Old Forge Creations

Views: 4,949

Keywords: pottery, clay, potterylove, potterylife, kilnfolk, ceramic, ceramics, contemporaryceramics, handmade, handmadepottery, createmakeshare, glaze, cone6, wheelthrown, wheelthrowing, potteryvideo, satisfyingvideo, oddlysatisfying, satisfying, oldforgecreations

Id: an6o3o_DXfU

Length: 8min 56sec (536 seconds)

Published: Wed Jan 18 2023