Low-fire unglazed Stull charts equivilents

[padjbarry]

Hello,

 

I'm a beginner potter, material scientist, and studying heritage unglazed architectural terracotta.  I first came across the Stull chart in the appendix of Linda Bloomfield's Colour in Glazes.
There's a really nice walk though of the original chart on youtube, and a javascript implementation that's used on glazy (https://derekphilipau.github.io/ceramic-chemistry-visualization/charts/). It's a little mad but awesome that we're still building on data graphed in 1912!

I'm working with unglazed bodies. Has anyone ever seen an equivalent graph or dataset for firing, porosity, vitrification or even colour for earthenware clays?
 

 

 

[Bill Kielb]

I think if manufactured clay the published data would be easiest for firing, porosity and color. I feel your pain though as it’s often difficult to obtain a firing curve. Stull is really more about glazes though. The traditional way to generate the data would be to fire test tiles of the particular clay in or close to you desired temperature and create your personal data library.

[Babs]

https://www.instagram.com/p/CeRYGg0pCdp/?igshid=YmMyMTA2M2Y=

Example.

[PeterH]

Perhaps of mild interest, a fully explored example of colour/porosity/shrinkage versus firing temperature.
SHAB - Shrinkage/Absorption Test https://digitalfire.com/test/shab

Note that the section "How do you decide what temperature to fire this terra cotta at?" is mainly concerned with glaze issues.

PS Do you include Code stone among "heritage unglazed architectural terracotta"?

[padjbarry]

Thanks Babs!  (I assume you meant the picture with the pallet of wild clays?)

Peter H - I have some samples of coadsetone in the dataset. It's a great material that's been relatively well studied compared to 'everyday' terracotta blocks and cladding. There's also something weird going on between big flat blocks and sculpture. The different sources suggest they degrade at different rates, but not always which one is worse off. It may be something to do with the different surface treatments, or simply luck of the draw. The surfaces is why I was thinking about stull - the degree of vitrification between refractory particles should be similar to the 

On the digitalfire data - yeah, that would be a single column in the table to make the chart. I am just a bit surprised no-one has done it yet. I know we flux things like crazy and kiln atmosphere has it's own role, but the alumina/silica ratio feels so fundamental to the changing porosity and propensity to bloating...

[PeterH]

Again only loosely related to the topic.

A post by Val Cushing on an Alfred University test to see if non-vitrified bodies are susceptible to freeze/thaw damage.  It's based on the ratio on two tests of porosity.
http://ceramicsfieldguide.org/pdf/val-cushing-handouts/special-extras/Cushing-OutdoorClay.pdf

https://digitalfire.com/article/outdoor+weather+resistant+ceramicsAn old newsgroup posting on the subject.
freeze-resistant ceramics--reply to vince http://www.potters.org/subject98627.htm

https://digitalfire.com/article/outdoor+weather+resistant+ceramicsAre any similar tests done for architectural terracotta bodies?

PS Oops should have looked at Digitalfire first
Outdoor Weather Resistant Ceramics https://digitalfire.com/article/outdoor+weather+resistant+ceramics
It is possible to perform a simple test based on the principle that a sample of fired ceramic boiled in water will absorb more moisture than one that is simply soaked. This is because for the former, the entire network is filled, for the latter only the pores. This test compares the cold soaking absorption or open porosity (C) of a clay with its boiled absorption or closed porosity (B) . The structural ceramic industry requires a C/B result of less than 0.78 (in products firing to more than 5% porosity) in order to pass CSA and ASTM specifications for outdoor use. If you are buying clay, your supplier should be able to tell you what the porosity will be at the temperature you plan to use it. If it is over 5% (as noted above) then you should be aware of the closed-over-open porosity also. If they sell the material for sculpture and know that customers are using it outside in freeze-thaw conditions, they should be able to tell you this figure.

 

Edited June 8, 2022 by PeterH

[padjbarry]

Loosely related to this topic, but definitely of interest!

A lot has been written academically on porosity and freeze thaw - less in terracotta, but brick, stone and concrete, and mortars all have similar issues. I'd strongly recommend Hall's book for the interested: https://books.google.com.vn/books?id=su_hmD6PrwEC&pg=PA79&lpg=PA79&dq=salt+swelling+brick+test&source=bl&ots=KelzHCY7VR&sig=ACfU3U13B6-aNtI7tXubPc6IgO1yVbhkpQ&hl=en&sa=X&ved=2ahUKEwiaxMO6sZ_4AhUxqlYBHc15BuIQ6AF6BAglEAI

 

Cushing's limiting ratio of 0.78 between cold soak porosity and boiled porosity is interesting. I wonder where he got it from? It sounds quite empirical. 

There is an ISO engineering standard for freeze thaw testing that is very direct - you alternate freezing a sample and spraying it with water until it thaws. Repeat 100x at about 45min per cycle for small samples. There's automated test equipment, but I know of one student who just takes her samples out the freezer at night and puts them back in each morning. It's slow, but easy

Likewise, there are various ISO standards for porosity and density measurement. Ignoring the more detailed but complicated/expensive methods like MIP and micro-ct, there's the three main ways of water measurement - cold soaking, boiling and vacumn impregnation. The last one is basically put the sample in a tank, suck the air out for 30min, add water and let sit for 15min. Boiling actually got taken out of the ISO last revision, and vacumn impregnation has been around for a long time - it was used by MacIntyre in his 1929 research on terracotta. It does get water into a few % more pores, is less likely to damage the specimen and has the advantage your not dealing with boiling hot limps of heavy material, so I've come to like it.

The terminology is a little confusing. Cushing's 'closed' pores are more like 'open cul de sac' pores, that are connected to the rest of the network, but have a closed end that air can't escape from. Normally in the academic literature closed pores is used to refer to pores that are basically sealed. One of the measures of freeze thaw damage is how much the open porosity of a block increases over time. It's not actually getting more porus, but the microcracking damage is reconnecting these sealed closed pores to the open pore network. 

There has been a lot of empirical work trying to predict freeze thaw propensity exactly, but I quite like one paper that suggests a rule of thumb based on the volume of pores of a key diameter (around 0.05mm iirc). The theory is that Pores smaller in diameter then this fill with water that freezes and expands, but the force from the tiny volume change isn't enough to crack the material. Pores much larger then this don't completely fill with water during normal conditions and so don't develop the full freeze-thaw forces.  Pores around that key diameter are small enough to fill up and big enough to generate cracking forces. 

Firing a piece to a higher amount of shrinkage might make pores that diameter a bit smaller and take them out of the danger zone, but it could also make pores that were larger and previously safe now small enough to be vulnerable. I think this is why developing a frost proof recipe is not straightforward.

 

Reality is more complicated of course: each new crack counts as a pore itself,  pores aren't neat cylinders; bottlenecks can freeze leading to big trapped pressures*; stress concentrations aren't idealised and geometry can massively magnify them; if a previously too small pore ends up surrounded by cracked weak material it might now be vulnerable; material strength varies; biofilms on the surface modify water flows to suit their needs and the continuous rain time varies by building location and exposure and is changing due to climate change. And finally, efflorescence - salt swelling cracking - has a similar but different set of parameters. Soaking samples in sodium chloride solution and drying might be an easier 'at home' test for overall durability.

 

*This seems to be the aspect Cushing's ratio is aimed at?

 

 

 

 

[Min]

I don't think it was Val Cushing who came up with the ratio. ASTM for bricks is behind a $60- paywall but you can find info indirectly on this link.  From page 4 under "Physical Property Requirements" the saturation coefficient test aka C/B ratio. If you then go to Table 4 "Physical Properties in Brick Specifications" the maximum saturation coefficient for the average of 5 bricks ranges from 0.78 up to 0.88  Just a hunch but I'm thinking that the 0.78 figure used by Cushing and Hansen is derived from ASTM or CSA requirements and they preferred the more conservative 0.78

[padjbarry]

Many thanks Min, that was the keyword I needed: there's a pdf here https://brickandtile.org/wikibrick/images/6/63/Relation_Between_Physical_Properties_and_Durability_of_Commercially_Marketed_Brick.pdf

 

That I've downloaded and attached to this post for records. It says the 0.78 comes from research in the 1930s, and while still the best test available, it's not that accurate and rejects a lot of good ceramics. It's also a bit of a nuisance for me- I've just done porosity tests using vacumn for 40 odd samples. I'd have to do it twice more for cold soak and boiling, with a week in the oven in-between to dry them out!

Relation_Between_Physical_Properties_and_Durability_of_Commercially_Marketed_Brick (1).pdf

[PeterH]

On 6/9/2022 at 4:49 PM, Min said:

I don't think it was Val Cushing who came up with the ratio

In case anyone wants to try them in searches: Ted Randall &  Dr Brownell were involved
http://ceramicsfieldguide.org/pdf/val-cushing-handouts/special-extras/Cushing-OutdoorClay.pdf