Why is water pressure increased in a plastic bag in a bucket?

[ImaginaryTango]

TL;DR

If I put clay in a bag of water, tie it up, put it in a bucket, the water permeates the clay - but why doesn't it do that in a big of water or just in the bucket alone?

I'm not a physicist or even a physics student, I'm a pottery student and there's a conundrum that's been puzzling potters in my area for a while. We've found that if a block of clay dries out, the fastest and best way to revitalize the clay is to put it in a bag of water, tie the bag so it's sealed, then put it in a 5 gallon bucket of water. In the diagram, that's D:

If we take the clay and put it in a bag of water, like in B, and let it sit for several days, the water has permeated the outside layers of clay, but only about 1" into the block. The same with C - if we take the clay and put it in a 5 gallon bucket of water, only the outer inch gets revitalized (or, probably a better term is remoisturized). But if we put the clay in the bag of water, the in the bucket, as in D, and fill it to the same level as we did in C, within 2 days, the entire block of clay is good for use - water has permeated through the entire block.

I've tried asking this in some physics forums and been told it's a chemistry issue, but when I ask in chemistry forums, I get responses that say something along the line of, "I think it's because..." but never a clear answer. (I've also had physics forums reject it because I couldn't include images in the question or had to ask the full question in the title and didn't know how to summarize it clearly in that short a space.) From what replies I have seen, it seems like it has something to do with pressure, but I don't understand why there would be more pressure in D than in C. (Or, even, since it's not even 18" of water depth, in B.)

Why does the clay get fully permeated in D but not in B or C?

 

[Dale]

Could it be a temperature difference? 18” of water is about 0.05 atmospheres, so it is some pressure but not a lot.

 

[ImaginaryTango]

Could it be a temperature difference? 18” of water is about 0.05 atmospheres, so it is some pressure but not a lot.

Good thought, but the water is from the same source. I know when I do this, I fill the bag, tie it off, then fill the bucket. And since it sits for about 48 hours, in my studio, it's at room temperature.

------ Added: -----
I wrote a long edit to this reply that got deleted - which may be just as well. I think it has to do with displacement. You have the water density, the density of the dry clay, and the density of the clay when it's infused with water. There's also the density of the clay around the edges when only those edges have been infused.

I think the density, from least to highest, would be:
1. Water
2. Clay
3. Infused clay
4. Clay edges infused with more than "normal" water. (And 3 & 4 might be reversed - not so sure!)

In B and C, when you put the clay in the water, it displaces water and the water and the outside water will have enough pressure to push water into the clay. As it does, the amount of water displaced goes down. Here's the part I'm unsure of, and I think the water infuses only into the clay to the point where the water pressure on the clay is no longer strong enough to infuse the clay anymore. (I've also tried to think through this and can see that the water infused into the outer edges of the clay may be equal to something to do with the difference of density of the clay and the density of water.)

The same thing happens like this in both B and C.

In D, you put the clay in the bag and seal it and you have the clay displacing water in the bag. Then you put the big in the bucket and the bucket water will apply enough pressure to try to make the bag shrink ot the size of the amount of water being displaced. So now the water in the bag is not only creating pressure to infuse the clay, but it has outside pressure that will push on the bag to try to make it as small a displacement as possible.

Now there's the inside pressure in the bag, that is pushing out against the bag, but also into the clay. But as long as it can continue to push water into the clay, it will not be pushing against the bag and against the bucket water pressure until as much water as possible has been absorbed by the clay.

So the displacement of the bag in the bucket is going to add extra pressure on the water inside the bag to reduce the displacement of the clay and that will be more pressure than what you get when you put the clay only in the bag or in the bucket.

 

[256bits]

So the displacement of the bag in the bucket is going to add extra pressure on the water inside the bag to reduce the displacement of the clay and that will be more pressure than what you get when you put the clay only in the bag or in the bucket.

The hydrostatic pressure at the clay surface will be the same whether the clay is enclosed in a bag, or free. While higher pressure could be of value, I conjecture it probably is not of great importance in this experiment, being of such low head.

The action of water infiltration is less sensitive to the water pressure, than it is to capillary action as the water moves into the clay pores, and from pore to pore as the clay is rehydrated.
A plastic bag is not impervious to gas or water, some types of plastic being less or more so depending upon composition and thickness.
Situations B and C are the same - a clay form surrounded by water.

D is different in that the plastic bag is employed as a permeable membrane allowing gas, water, perhaps ion exchange between the fluid within the bag and that without. ( within and without should be antonyms, and if not, I just made then so ).

Question is what is the difference between the two separate waters that would allow more water infiltration into the clay. Has the pH level changed due to dissolution of clay salts or compounds rendering the clay less hydrophobic in a general sense? CO2 content, O2 content?

I am throwing out some chemistry. Hopefully some will stick. What the actual mechanism truly is might be found with testing the water content of the interior. ( personally, I lean to pH, as clays are more basic than acidic, but that is only a hunch ).

 

[Hill]

Did you actually try C?
In several articles about this process I've found, they add a bit of water inside the bag to moisturize the clay and add water outside the bag for pressure.
E.g.,

 

[A.T.]

Question is what is the difference between the two separate waters that would allow more water infiltration into the clay

One could do a version of C, but not with initially clean water, but water that already has as much as possible clay suspended in it.

 

[ImaginaryTango]

Did you actually try C?

Small point - what I'm doing is basically what a lot of the local potters do, so you could say I'm following "tradition" or "local lore."

I haven't, but I know others who have. I also have not tried different amounts of water in the bag, since others have said, "This works," and I tried it and it did - filling the bag with water until it comes up to near the top of the clay, maybe even over. The bags are the bags the clay comes in, so there's often not a lot of extra space in the bag. In the local community, we don't put any holes in the clay itself, like she does in the video.

I check my bags after 48 hours and it's in good shape for use. Again, I went by what others said - give it a couple days. It could be just fine after 24 hours, I just haven't checked it that soon.

A plastic bag is not impervious to gas or water, some types of plastic being less or more so depending upon composition and thickness.

I'd like some clarification or expansion of this point before moving on, since it's confusing me. When I fill the bag with water, and I've tested this, I don't see any signs of water permeating the bag. Before using a bag, I fill it with water, tie it off, dry it with a towel, and leave it in a dry 5 gallon bucket, check later to see if there is any water on the bag or in the bucket. Then I move it around and leave it again. (A few times I did this, thought a big was in good shape, only to find a pinpoint leak that wasn't leaking due to the position of the bag in the bucket.) While I don't think this applies to anything in the discussion, I have found that if there is tiny leak in the bag itself, it doesn't work. I'm not disagreeing with you - I'm asking for more information about this.

Question is what is the difference between the two separate waters that would allow more water infiltration into the clay. Has the pH level changed due to dissolution of clay salts or compounds rendering the clay less hydrophobic in a general sense? CO2 content, O2 content?

I don't see how there could be any difference between the two waters at the start. I have a sink in my studio with a curved high water spout. I put the clay in the bag, fill the bag with water until it comes close to covering the clay, then tie it off. Then I put this in the bucket and fill the bucket. So the bag and bucket are filled from the same water source, with less than a minute between them. As for any changes in the water, yes, there could be something from the clay that dissolves within the bag. I don't know and I wasn't thinking in terms of chemistry, so I don't have pH strips to test the water. And, yes, maybe there's a change in the bucket water. I could see that maybe the outer one releases air trapped in that water over the next few hours, and in that bag, any released air might stay in the bag or might permeate the bag and escape.

But if something is dissolving from the clay and into the water, I would think that would work the same if I just used B or C. (And, you're right, they're the same - but I did try both ways and neither worked.)

Reading what I just quoted from you and this part over again, it raises a question in my mind about possible gas in the glay as the clay dries out. I don't know how that would interact with anything in this situation, though. I can imagine a few things, like maybe for some reason, in B and C, as it dries out, and maybe air makes its way in where the water was (wondering about this - since there's really no noticeable shrinkage as the clay dries out - there is notable shrinkage when I throw a piece and leave it out in the open air to dry out. But that's out in the open air for a good while. (Again, I know my experience in this is quite limited - I'm just adding points like that in case they matter to those who know physics or chemistry or both.)

I am throwing out some chemistry. Hopefully some will stick. What the actual mechanism truly is might be found with testing the water content of the interior. ( personally, I lean to pH, as clays are more basic than acidic, but that is only a hunch ).

I'm fine with that - my whole thing about displacement and pressure was an attempt to throw out what I thought might be involved. And I may not have quoted or replied to your point about capillary action, but I think that's an important point: What is creating an effect on capillary action in D that isn't there in B or C? I can test pH, but I don't know what else I can test.

I'm thinking if I do testing, that may be tricky, considering how much clay I used over the holidays, for making presents, and what's left. Seems to me I need to take one type of clay, cut the brick in half, put one half in something like D and one in something like B and test the pH in them after, what, maybe 12 hours? That way we'd be sure that there is or isn't a pH difference between B and D. (Don't see a need to test C, since we're talking about the change inside the bag.)

 

[ImaginaryTango]

One could do a version of C, but not with initially clean water, but water that already has as much as possible clay suspended in it.

I think you're thinking along the same lines as @256bits with that - what makes the water different chemically. Playing the devil's advocate here, but I can think of 2 points that might indicate that the used water is an issue is that I would think that leaving the clay in for 48 hours or so would lead to clay suspended in the water in C. And that's something we've discussed locally - whenever we've done B or C, even leaving it in for extra time, the water only permeates into the edges (and maybe 1" inwards) in the clay.

 

[Dale]

If you have a sous vide you could test temperature dependence. Room temperature and residential water temperature can fluctuate. I don’t think that variable has been adequately controlled

 

[A.T.]

I would think that leaving the clay in for 48 hours or so would lead to clay suspended in the water in C.

Yes, but C doesn't have the same suspended clay concentration as D, where the suspended clay is prevented from dissipating throughout the whole bucket by the bag.

With regards to B vs D: Is the bag really so full of water, that it has the same shape as in D, as shown in the picture? Or does it sag to the bottom, and stick to the clay at the top?

 

[Gavran]

https://ceramicartsnetwork.org/cera...monthly-article/quick-tip-reconstituting-clay

 

[jbriggs444]

What about the possibility that capillary action is responsible?

By itself, capillary action will tend to draw water into the clay. This will tend to expel any entrained air. If the clay is entirely immersed, there is nowhere for the entrained air to go. It is trapped and will be under some pressure. At best, it can attempt to bubble out the top. But surface tension will resist bubble formation. If, on the other hand, the clay has a surface exposed to air, then the entrained air has an exit pathway. Capillary action can then fully displace the entrained air.

The plastic bag is helpful because it ensures that air inside the bag will reach 100% relative humidity. The exposed clay will not dry out.

 

[256bits]

I'd like some clarification or expansion of this point before moving on, since it's confusing me. When I fill the bag with water, and I've tested this, I don't see any signs of water permeating the bag.

Pore size of plastic film is very small, much more so than the experiments that show the surface tension of water not allowing water to escape say from an eye dropper.

some literature.

https://www.researchgate.net/figure...tic-films-used-in-packaging-17_tbl1_225207929

Polymers and plastics are permeable and their barrier properties may impact their suitability for a specific application. In packaging, for example, ineffective barrier properties for plastic films may render the enclosed product vulnerable to surrounding environmental factors, such as water, humidity, and oxygen, including its future storage.
https://www.intertek.com/polymers-plastics/barrier-properties/
The main gases that affect the shelf life and packaged stability of most products are oxygen and water vapour and so we provide Water Vapor Transmission Rate (WVTR) via industry established methods ASTM D1653, Water Vapor Transmission of Organic Coating Films and ASTM E96 Water Vapor Transmission of Materials. We can also provide testing to ASTM F1249, Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor and ISO 15106-2, Plastics - Film and sheeting - Determination of water vapour transmission rate.

The Oxygen Transmission Rate (OTR) is an important determinant of the packaging protection afforded by barrier materials particularly for film, sheeting, laminates, coextrusions, or plastic-coated papers. We measure the OTR via ASTM D3985, Oxygen Permeation of Flexible Barrier Materials Using a Coulometric Sensor ASTM D3985, ISO 15105 to determine the permeability coefficient, permeance and transmission rates.

 

[256bits]

One could do a version of C, but not with initially clean water, but water that already has as much as possible clay suspended in it.

I thought of that also.
The water-water plastic film can be important for molecular water and gas transport, more so than an air-water plastic film.
Maybe, depending upon what's happening.

 

[256bits]

What about the possibility that capillary action is responsible?

By itself, capillary action will tend to draw water into the clay. This will tend to expel any entrained air. If the clay is entirely immersed, there is nowhere for the entrained air to go. It is trapped and will be under some pressure. At best, it can attempt to bubble out the top. But surface tension will resist bubble formation. If, on the other hand, the clay has a surface exposed to air, then the entrained air has an exit pathway. Capillary action can then fully displace the entrained air.

The plastic bag is helpful because it ensures that air inside the bag will reach 100% relative humidity. The exposed clay will not dry out.

I mentioned capillary action in post 4. It is how the water moves from pore to pore in porous medium, much like paper sucking up water. Clays will do the same, as well as sand at the beach, up to a point where the capillary pressure matches gravity. The surface tension of water will draw water along a surface, and thus from pore to pore.

 

[Hill]

Why the bag has an effect upon that is the question.

What is the evidence that it does?

 

[DaveC426913]

within and without should be antonym

That is a valid application of without, yes.

 

[DaveC426913]

I'd kind of like to see the OPs test results duplicated. It would rule out experimental error and also provide some numbers to work with.

Or is this already a well-known phenomenon?

 

[256bits]

I'd kind of like to see the OPs test results duplicated. It would rule out experimental error and also provide some numbers to work with.

Or is this already a well-known phenomenon?

By that video from post 5 it appears to be a known phenomena.

 

[Hill]

By that video from post 5 it appears to be a known phenomena.

The video in post 5 does not show the experiment C.