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Dave K

Absorption

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Would someone explain absorption in relation to clay. I use stoneware and porcelain and fire to cone 6. Any information and advice about how it effects to pottery making process would be appreciated. 

Thanks.

 

 

 

 

 

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Dave:"absorption in the pottery process" covers a broad spectrum

1. While forming, in particular throwing: porcelain and stoneware absorb water differently. Porcelain is primarily kaolin, which is a 1:1 clay particle that holds water only on it's surface. Which means when the clay approaches maximum absorption, the body becomes weak and the piece becomes more prone to collapse. Stoneware  has large amounts of ball clay, which is a 2:1 clay structure, which holds water on and in its inner platelets. Stoneware actually absorbs water, which is why it takes much longer to dry. Kaolin only holds water on it's surface, so it dries much faster.  So this is the first step in which a potter deals with absorption.

2. Bisq pieces and green ware also have absorption rates: both are usually addressed with allowing time for it to dry.

3. Absorption in relation to vitrification is usually what potters refer to as absorption values. Absorption becomes an issue only when a piece is designed for functional use. It can also become an issue if pieces are subject to outdoor conditions, are used as tiles in wet areas. In these cases, absorption is gauged by the volume of water absorbed by the fired piece. Porcelain at cone six if properly fluxed should be under one percent. Stoneware at cone six should be under two percent. There is a variety of thought and theorem, and some disagreements about acceptable levels.

There is a myriad of issues that effect absorption that include firing schedules, clay formulation, and  glaze applications. So you will get a variety of answers pending how various potters view each as the fix. Personally I gauge and test absorption on a fired unglazed piece. Weighing the piece after firing, and again after a two hour soak in boiling water (patted dry). The difference in weight determines absorption, if any. Unless you mix your own clay, discussions on particle distribution and flux molarity are moot. You then have to rely on the maker to provide this data: the other option is firing 1/2 to 1 cone higher than recommended to increase the glassy matrix inside the piece. However, you must also test to make sure your clay and glaze of choice can handle the extra heat: most can.

so which absorption point is causing you grief?

Nerd

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Absorption is usually rated to the absorption of a clay body in the glaze fired state. Imagine an unglazed area of a an earthenware pot, that area would allow more water to be absorbed into the clay than a clay body that is fully vitrified at ^6 which would be slightly more than a body fired to ^9 or 10. At the same time, the porcelain body that vitrifies at ^6 is going to absorb less water than the stoneware at ^6. This is an over simplification, as many factors  are involved including the vitrification of the body, and the clay type. There are ways of measuring absorption, as in soaking a piece of glaze fired unglazed clay in water and comparing the dry weight to the wet weight. Most of these things can be covered in books like Mastering Cone 6 glazes, or even by talking to your supplier.  Big deal, maybe not, maybe so, but considering that a lot of wear and tear on functional  pottery occurs from washing/dishwasher, the absorbance will effect the possibility of crazing in the glaze, thus making the pot less functional and somewhat unhygienic 

 

Again all an oversimplification,

best,

Pres

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Thanks for the replies. Nerd, I think it is point #3 that is my problem. I will try not to make this too long. I use Standard Ceramics clay and have been making vases with 182 which is cone 6-10. My wife put flowers in one and it ended up leaking. Upon testing more of my vases I found that they almost all leak. Doing some research I find that 182 is not vitrified at cone 6 and that glaze will not usually prevent leaking. It makes me wonder what good is a cone 6-10 clay. I am in the process of changing to a different clay body that is a true cone 6 clay. In looking at various ones Standard has to offer I see the absorption rating and wondered what it actually means in the overall picture of making vases.

I also tried 365 and 213 porcelain. The 365 did not leak but the 213 did, and they are both rated at cone 6.  They were both fired to a cone 6 by the pyrometric cones. It leaves me more than a little confused.

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I used 182 way back when I first started in my own studio. I found much the same problem. I have come to the point that now I only use clays that are ^-5-6 for that reason. I am now using their Hazelnut Brown, and a white stoneware-630 that is basically a ^6.  I had wondered where your question came from, but it is quite clear now. Long range firing clays are problematic for funtional ware potters, especially if you are hoping to have pieces serviceable for many years.

 

best,

Pres

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1 hour ago, Dave K said:

I also tried 365 and 213 porcelain. The 365 did not leak but the 213 did, and they are both rated at cone 6.  They were both fired to a cone 6 by the pyrometric cones. It leaves me more than a little confused.

Standard lists 213 at 1 point 7% absorption at ^6, 365 at 0 point 6 at ^6. There is usually a fudge factor of plus or minus 1% Do your own absorption tests to determine absorption figures for your kiln and how you fire. Also, how evenly does your kiln fire? Not uncommon to have a full cone difference between the bottom and/or top of the kiln and the middle section in manual controlled kilns or kilns with just 1 thermocouple. If this is the case with your kiln then the vases need to go in the hottest section of the kiln.

I would also fire some unglazed little pots with flat bottoms, put cones beside them in the kiln. Fill with water, put them on newspaper for a couple days and check the paper for wrinkling/dampness. If it wrinkles the pots are weeping.

For vases I would be looking for under 1% absorption for the cone you are firing to.

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I picked up some 563 and tried it this afternoon. It was a little soft but easy to throw. I will see how it fires. I will also check with my supplier to see if they have 630, if not I will have them order some and give it a try.

Min, I have an L&L kiln with 3 thermocouples and it's pretty much the same on all levels at cone 6 but not at cone 04. Is there any "wiggle room" for a cone 6 clay to be fully vitrified? If the pyrometric cone is at 2:00 is that close enough and if not am I better off bumping the hold time up a little and risk being over fired?  Are you saying that if the absorption rate is high enough that a vitrified piece will leak?

Thanks to everyone for the suggestions and help.

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Ideally we want absorption under 2%, otherwise it can weep and it's not vitrified. Half a cone may or may or may not matter- it all depends on the clay body. If your glaze can handle it, go for a full cone 6 by adding a hold time at the end. Start with 5 minutes and increase by 5 minute increments, or put on a 30 minute hold and watch the cones to see when it's done and shut it off manually one time. But that means being there to monitor the end of the firing. Shouldn't take more than 10 minutes hold

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Agreed with Neil, I would also try for a full cone 6. For large standard cones it’s tip touching the shelf for a full cone 6. With self supporting cones it’s tip level will the top of the wedge shaped base. (If you go tip touching shelf with the self supporting cones it’s only a few degrees more so that is okay too.) Some clays have more wiggle room than others. In my experience the closer to 0% porosity the less you can overfire without running into the occasional bloat or slump. I don’t have enough experience with US clays to know how much you can overfire the Standard Clays. I looked up the Standard 563, it’s listed as 1% @ ^6. But don’t forget that fudge factor of + or - 1%, gotta do your own tests to see if it’s okay for pots holding water.

Sounds like an uneven kiln is not a problem, good. I didn’t know what you had so was trying to cover all the bases with my earlier comment.

Couple thoughts re what is vitrified and if it will leak. Guess it depends on how you define vitrified and mature. I go with if the fired clay is impermeable to water then it is vitrified. (ie it doesn’t leak at all). Mature is not necessarily vitrified. Example would be practically all earthenware clays. They mature roughly somewhere between 04-2 but the porosity (how much water they soak up) is never going to be near zero, generally in the 10%+ range. Yup, they are mature, nope they are not vitreous. So to answer your question, your ^6 clay likely isn’t vitrified if it isn’t fired to maturity. Your wide range 6 - 10 clay is not fired to maturity at ^6 therefore no way will it be vitrified nor mature.

Edited by Min
correcting spellcheck (overfire isn't a word?)

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One test you can do to see if your clay has vitrified, is to fill it with water and let it sit of a surface all night. In the morning , if there is a ring of moisture at the base, the clay is not vitrified. When working with outdoor freeze proof clay, there are a few differences of opinion on what rate is preferred. Nothing over 3%. Some say 1.5%

Marcia

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I've been testing out a clay body that the manufacturer states is cone 4-10 which is such a huge range. I've taken it to cone 6 which is what I fire to and it has 5% absorption, which is ofc way too much for functional ware. But I had a thought when you test for absorption, do you test the clay with or without glaze? I mean I figured it would be best to replicate what people would be using so I tested it glazed in a manner I would glaze the piece.  

 

What do you guys do?

 

Thanks,

Spring

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54 minutes ago, spring said:

I've been testing out a clay body that the manufacturer states is cone 4-10 which is such a huge range. I've taken it to cone 6 which is what I fire to and it has 5% absorption, which is ofc way too much for functional ware. But I had a thought when you test for absorption, do you test the clay with or without glaze? I mean I figured it would be best to replicate what people would be using so I tested it glazed in a manner I would glaze the piece.  

 

What do you guys do?

 

Thanks,

Spring

Test without glaze. 

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Test without glaze.  If the glaze has a single pinhole that goes to the body, or if the glaze is crazed....... any liquid inside it will reach this body.  Then ... if the body is not impermeable to liquid passing thru......... it will eventually weep/sweat/leak.

If you plot the percentage absorption on a graph relating % absorption to firing cone you are going to get a sort of a "curve" for any clay body.  It is the shape of this curve that tells you a LOT about how that body will perform relative to absorption over a certain firing range.  A steep slope in any area tells you that a slight change in the cone the work is fired to will have a DRAMATIC change in the absorption of the body.  A more gradual curve slope tells you that there is more tolerance for firing variations.  

Usually the curve plotted is not in any way symmetrical..... meaning that the slope of the line on one side of the minimum absorption point will not reflect the shape of the line on the other.  As a clay gets overfired, the % absorption will increase a bit as the body "fizzes" and voids start to show up.  This side of the curve usually is a less steep slope area.  (Of you also plot the strength of the clay in this overfired area...... the strength goes down steadily.)

The steeper the drop to the minimum on the lower side of the line, the more specific the firing cone of the clay is.

Clay bodies that are intended to be vitrified typically have a reasonable firing range of about 3 cones max.  No such thing as a cone 6-10 body... and the new trend stating "cone 4 to 10" should likely be challenged legally as "misrepresentation" and not meeting the dictates of the law of merchantability.

best,

...........................john

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