Absorption for functional ware

[HenryBurlingame]

What is the highest absorption % you all think is OK for functional ware? I am looking to try some new clay bodies (for cone 6), but a lot of the ones that look nice to me have absorption rates that seem a bit high for functional ware...

[Kelly in AK]

Gray area for some of us old schoolers. I stick with < 1.5% as solidly safe. I was taught (30 years ago) anything under 3% is passable but less than ideal. That’s where the leaks begin to show. Under 2% wont leak. Over the years popular opinion has reduced these numbers, probably creating stronger more reliable ware.<1% is a more common goal for most people now. Glaze fit has developed in tandem in the modern craft potters’ craft which skews results a bit. A well fitting glaze can strengthen and protect a pot that is not fully vitrified for years. I have some mass produced diner plates that are proof. 

Firing a cone higher (or a soak) can sometimes get you the absorption level you want without destroying the glazes.

Edited July 21 by Kelly in AK

[HenryBurlingame]

Interesting. Less than 1% would basically eliminate most cone 6 stonewares all together… nearly every single one from the suppliers in the same state as me (seattle pottery supply and clay art center)  are listed as 1% and over.

Edited July 21 by HenryBurlingame

[Magnolia Mud Research]

ZERO!  

grandma don't like water on the piano!  

if the pot leaks at cone 6 then fire to cone 7 or 7 and half.  
I started ceramics using  a Cone 06 clay body that was fired to cone 2  to produce pots that did not leak.   At cone 06 one is depending on the glaze to keep the water inside the pot;  any glaze crack means grandma uses that pot for an outdoor flower pot or for the next trash pickup.   

LT
 

[HenryBurlingame]

How many cones higher can you usually take a stoneware clay before you start getting issues?

Edited July 21 by HenryBurlingame

[PeterH]

1 hour ago, HenryBurlingame said:

Interesting. Less than 1% would basically eliminate most cone 6 stonewares all together… nearly every single one from the suppliers in the same state as me (seattle pottery supply and clay art center)  are listed as 1% and over.

Which clay are you using?

A quick glance at Seattle suggests that they are giving absorption at mid-range rather than at cone 6, with the odd one <1% at cone 5.

Digitalfire gives information on doing your own shrinkage tests. Maybe worth trying at cone 6 and "extended" firing schedules.
https://digitalfire.com/test/shab

PS https://seattlepotterysupply.com/pages/water-absorption-in-pottery-clay
When you are making dinnerware (plates, cups, bowls) that will come in contact with food you would want a clay with the lowest absorption rate possible(<.5%) This is because clays with a low absorption rate will not absorb water even if there is a bad fit between the clay and glaze (cracks in the glaze). 

When making work that will hold liquids but not necessarily food (vases, bird baths, fountains) an absorption rate can be slightly higher (.5-3%).  While the clays have a higher absorption rate they should not absorb enough water to seep through the piece onto the table.

Might be worth asking them which bodies -- when fired to cone 6 -- meet their recommendations for dinnerware.

[Marilyn T]

To find a Cone 6 clay that I was okay using for functional ware I did the following:

1.  fired it unglazed to Cone 6, Cone 6 1/2 and Cone 7.    My results for absorbancy were:   2.54%. 1.5% and 1%.  The ware got progressively coarser and smaller but not bloated as I fired higher.  

2.  I then filled one cup from each firing with water and left for 24 hours on a piece of water absorbent paper.  Results:  none of them leaked.

3.  I then dumped out the water and microwave tested them (using a pyrex cup filled with water as my safe).   My clay was iron rich so I did get warming but there was not a discernible difference between the 3 cups.  I was able to hold all three cups in my palm but they were on the hot side of warm.  The handles were neutral.  

4.  I put three difference glazes on the cups = 9 total and fired them to the same cone #'s.  I liked the results of the Cone 6 firing the best.

5.  The subsequent oven to ice water tests didn't reveal any crazing in the glazes.   

Decision:  Because  there wasn't a difference in the leakage or generated heat I decided to go with Cone 6 based on the appearance of the ware.   And yes I know the recommended absorbancy rate is .5% but I am comfortable (as a hobby potter) with labeling this ware functional with a note that it will warm up in the microwave.   Feel free to add your thoughts to this.  😊

[Min]

Getting actual absorption figures from a boil and soak test is quantifiable and repeatable using the same conditions. I do the test below but add an 8 hour simmer to the beginning of the test then leave the samples in the water for the remainder of the 24 hours.

From this link:

"The weight calculation test is more specific. Weigh the fired bar on a gram scale and record the result. Soak the bar in water for 24 hours, pat dry, then weigh again and record the result. Subtract the first weight (dry bar) from the second weight (water soaked bar) to get the weight of the absorbed water. Divide the weight of the absorbed water by the original dry weight and move the decimal two places to the right to find the absorption rate.

Example: Original bar weight of 4.2 grams is subtracted from soaked bar weight of 4.6 grams giving you an absorbed water weight of .4 grams. Divide .4 by 4.2 which equals .095 making absorption 9.5%."

I don't think a 0% absorption is possible with many stoneware bodies as bodies will slump, bloat or become excessively brittle  well before reaching 0%, I'm sure many would never be able to reach 0% porosity no matter how high they are fired. Overfiring a body greatly can also increase porosity, the fired matrix effects porosity, when claybodies start to break down porosity increases. Ruling out everything with absorption over 0% isn't necessary. Absorption is a result of claybody chemistry plus heatwork just as mature glazes are a result of chem plus heatwork.

Hands on testing for many years, my main claybody is consistantly in the 1.5% +/- 0.15 with no issues using some of the pots daily. Earthenware  bodies are fine with their high absorption figures as long as the (functional) pot has a well fitting glaze covering it entirely.

Another factor is open cell porosity vs closed cell porosity, especially if making work subject to freeze/thaw situations.

Edited July 21 by Min
clarity

[HenryBurlingame]

Great info, thanks so much you all, this is very helpful!

[Kelly in AK]

16 hours ago, HenryBurlingame said:

How many cones higher can you usually take a stoneware clay before you start getting issues?

I find this a wonderful question. BMix-5, which I use regularly is described as 2.3% absorption at cone 5. It is absolutely tighter at cone 6, where I fire it. If cone 7 is down it wil start slumping. I believe different bodies have different functional ranges. My local clay goes from porous to vitrified to slumping in a cone and a half. (04- porous, 03- vitreous, 02 tipping - slumping, followed shortly thereafter by bloating. 

[Bill Kielb]

5 hours ago, Kelly in AK said:

BMix-5, which I use regularly is described as 2.3% absorption at cone 5. It is absolutely tighter at cone 6, where I fire it. If cone 7 is down it wil start slumping.

A point well made. If one looks at the curve of a fired body, the density often starts to decrease rapidly beyond its top rated cone. The density curve often falls off sharply after rated cone. Producers likely give some leeway and I am guessing not all same composition bodies react exactly the same or are perfectly homogeneous.  Likely often a good thing to keep in mind when pushing clay products in this fashion. 

[Hulk]

"I believe different bodies have different functional ranges."

Agreed. The two white stoneware clays I've used (and liked), and the red stoneware I'm using now can take more heat work.
Several other stoneware clays I'd found to be much more sensitive to heat work.

[Mark C.]

Cone  bodies 5+6 really need testing . Its just the way it is. To  hot and the body will bloat too cool it leaks-testing testing

Disclaimer -no real cone 5-6 ever done. Plenty cone 06 in early 70s but all cone 10 since. 

Edited July 22 by Mark C.

[neilestrick]

4 hours ago, Mark C. said:

Cone  bodies 5+6 really need testing .

Respectfully, all clay bodies need testing. Cone 10 doesn't guarantee fewer problems than cone 6. I've seen cone 10 bodies leak and bloat, too.

[Mark C.]

1 hour ago, neilestrick said:

Respectfully, all clay bodies need testing. Cone 10 doesn't guarantee fewer problems than cone 6. I've seen cone 10 bodies leak and bloat, too.

Me too , never did  say they where bullet proof -all bodies need testing no matter what the cone. I tested mine long ago. I too have seen cone 10 bodes flaw -leak and bloat . In fact I have two car kilns loads of porcelain bloat long ago and sent all the wares back to laguna to the owner-I posted that story here awhile back. Been there done that

[Bill Kielb]

All clays have some form of this characteristic regardless of cone, as the body tightens at some point any further change has a significant effect. Ceramic Materials workshop has a course in profiling your clay - might be interesting. The example they post on their website seems to follow most intuition though - just very specific to the claybody.

For me, Interesting to note the manufactures suggested firing point (yellow) and the ASTM standard (red) in relation to absorption. Also interesting to note absorption can be a bit of a lagging indicator with respect to density.

Also interesting, the linear change vs temperature and maybe more fascinating the measured CTE at various temperatures. Maybe Looks like a cool course to me.

Also sort of cool, max bisque absorption point!

 

Edited July 23 by Bill Kielb

[HenryBurlingame]

I will check it out, thanks for the heads up Bill, it does sound very interesting. Love the graph you posted for B-mix, thanks! Surprised to see so much absorption difference between cone 5 and cone 6... maybe those clays I'm afraid of from Seattle pottery supply will be just fine if I fire to cone 6. Its looking like my studio is going to be in need of a little test kiln in the not so distant future lol.

Edited July 23 by HenryBurlingame

[davidh4976]

One thing I've noticed about references to an ATSM standard is that the standards I have been able to find only apply to ceramic tile and not to functional ware like cups or vases. I don't believe there are ANY formal specific standards for functional ware absorption. Anytime I have entered a discussion about standards that apply to functional ware and not tile, I've asked for anyone to cite the standard and the section. I'm still waiting for that. 

Personally, I like to shoot for less than 1%, but believe there is a gray area somewhere between 1% and 2% that is probably ok. Again, that's just me personal rule of thumb.

[Min]

3 hours ago, davidh4976 said:

One thing I've noticed about references to an ATSM standard is that the standards I have been able to find only apply to ceramic tile and not to functional ware like cups or vases. I don't believe there are ANY formal specific standards for functional ware absorption. Anytime I have entered a discussion about standards that apply to functional ware and not tile, I've asked for anyone to cite the standard and the section. I'm still waiting for that. 

Personally, I like to shoot for less than 1%, but believe there is a gray area somewhere between 1% and 2% that is probably ok. Again, that's just me personal rule of thumb.

Perhaps we need to look beyond the ATSM. From this link:

"Who creates standards? There is an entire alphabet soup of standards organizations around the globe prescribing tests. For ceramic artists in the US and Europe, there are at least six such organizations. The American Society for Testing and Materials (ASTM) is the main one, along with the American National Standards Institute (ANSI). In Europe the British Standards Institution (BSI), the German Institute for Standardization (DIN), the International Organization for Standardization (ISO), and the Committee for European Standardization (CEN), all write ceramic standards."

I haven't looked into the BSI, DIN, ISO or CEN. Perhaps there are functional ware standards within one of those?

[akilpots]

21 hours ago, Bill Kielb said:

All clays have some form of this characteristic regardless of cone, as the body tightens at some point any further change has a significant effect. Ceramic Materials workshop has a course in profiling your clay - might be interesting. The example they post on their website seems to follow most intuition though - just very specific to the claybody.

For me, Interesting to note the manufactures suggested firing point (yellow) and the ASTM standard (red) in relation to absorption. Also interesting to note absorption can be a bit of a lagging indicator with respect to density.

Also interesting, the linear change vs temperature and maybe more fascinating the measured CTE at various temperatures. Maybe Looks like a cool course to me.

Also sort of cool, max bisque absorption point!

they released a bunch of clay profiles i think over 40 clays. 

https://ceramicmaterialsworkshop.thinkific.com/courses/take/commercial-clay-body-profiles/texts/51630181-introduction

[Kelly in AK]

The CMW clay profiles website is very cool. You do have to make an account to view, but it’s free of charge. I really appreciate them making it freely available, lots of work went into producing that data.

Now, to a fun point. I like to dog on any clay that’s billed as cone 6-10, because there’s always an element of misrepresentation there. Someone is trying to sell clay to the widest audience possible, miracle clay, can do it all. And there ain’t no such thing. So I thought. To my surprise, there are a couple that come pretty close. Continental Clay Company’s buff stoneware and Standard’s 259.

So when you think of the range a clay can fire in and be considered vitreous enough for tableware these are interesting. Of course there are a lot of other things we expect our clay to do, like be plastic but not crack, and fit the glazes we like, and look how we want it to look. So I’m probably not done dogging on the idea of cone 6-10 clay yet, but maybe have to be a little more careful with my pronouncements! 

[High Bridge Pottery]

It's certainly expensive to try and read these standards. I tried to get access through work but although they have access British Standards are very cagey with their intellectual property.

 

As far as I can tell absorption figures generally define what the clay is and how you can name it, not how functional it is.

Things like BS 4034 "This British Standard specifies requirements for resistance to water absorption and crazing for vitrified hotelware which determine that the description is applied correctly to the product."

 

The only real functional aspect of porosity seems to be does that water cause expansion of the body and make it craze? BS EN 13258 "Defines two methods of test: Method A and Method B, to determine the crazing resistance of glazed ceramic tableware articles in contact with food having water absorption higher than 0.5 %. Method A is a test for resistance to crazing due to moisture expansion. Method B is a test for resistance to crazing due to thermal shock."

 

As the above standard seems to say, anything under 0.5% you don't have to worry about moisture absorption causing crazing.

[davidh4976]

Crazing is prohibited for commercial food service by various regulations. In the US, most States have adopted FDA guidelines that cover this and it is therefore in the food code for the States. I believe the EU also has a crazing standard for commercial food service.

What BS EN 13258 is saying is that if your absorption is less than 0.5%, you don't use the  described methods for testing crazing.  If your absorption is more than 0.5%, BS EN 13258 provides two methods of testing for crazing.  It does not say whether your functional ware should be less than 0.5%.  The quest continues...

 

Edited July 25 by davidh4976

[Kelly in AK]

On 7/24/2024 at 7:24 AM, High Bridge Pottery said:

As the above standard seems to say, anything under 0.5% you don't have to worry about moisture absorption causing crazing.

There are a few other things. At 0.5% you have little  worry about the ceramic absorbing liquid and becoming a media for bacterial growth, even if the glaze is crazed. The density of ceramic correlates pretty closely with absorption, so at 0.5% the material tends to be as strong as it will get. Finally, you’ll have little concern absorbed water will cause ware to get unduly hot in a microwave.

So the 0.5% standard, by whatever organization proffering it, is checking a lot of boxes. I think of it as the minimum a third party would sign off on to guarantee nothing will go awfully wrong. I think of it practically as an extremely high standard unless one is operating in a very controlled environment.

As @High Bridge Pottery says, there is a difference between absorption figures and how useful a pot is, or how a pot is useful. There is significant space between pottery that behaves as expected by most people and pots at 0.5% absorption. Many pots scarcely approach this standard and are used regularly in kitchens worldwide, pperforming well as expected. There are caveats, variables, exceptional circumstances,  personal tastes, and even fads that influence what gets named as functional pottery.

We haven’t even discussed porous ceramic with such impeccable glaze fit it checks all the right boxes except absorption. 

Edited July 25 by Kelly in AK
Clarity

[High Bridge Pottery]

I am not sure there is a different test for ware under 0.5% but I will keep looking.

It seems the only thing absorption can standardise is if you call it vitrified, as Kelly says you can make perfectly functional tableware that's porous and passes any food safety test.