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

[A.T.]

...when you say the bag prevents or slows some kind of exchange - but wouldn't it be that it allows or speeds up an exchange, not slowing it down?

Why? The bag foil prevents/hinders transport.

 

[ImaginaryTango]

Why? The bag foil prevents/hinders transport.

Okay - just trying to straighten it out in my brain.

 

[Dale]

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.

This sounds like the most likely hypothesis.

It could be tested by comparing a bag with trapped air and a bag with all air removed. It would also be interesting if the bag without air got an air bubble from the air originally inside the clay.

 

[ImaginaryTango]

This sounds like the most likely hypothesis.

It could be tested by comparing a bag with trapped air and a bag with all air removed. It would also be interesting if the bag without air got an air bubble from the air originally inside the clay.

I'm really glad you posted this and shared the quote from @jbriggs444. I did read his post, but maybe, since I was reading a lot of posts, I missed some details and that may be the answer (or the best one we can get without a lot of experiments).

I think, when you put the original material in the quote together with your suggestion about testing with and without air, that gives me something I can test. I will, and I'll report back here - but it'll take me a while since I can't really test it until I have dried out clay.

Thinking this through, which I usually do as a thought experiment when someone makes a suggestion, I'm wondering if there might be a reason it might work better the opposite way - that maybe, if there is air in the bag, and I put it in the bucket, the water pressure against the bag helps make sure the air all goes to the top, possibly leaving the block covered when it might not be 100% submerged in just the bag.

But then, that would make that just like C, wouldn't it?

 

[A.T.]

I fill the bag to almost cover the clay, but not completely.

I guess I took the diagram too literally then. In this case I agree with @Dale that the mechanism proposed by @jbriggs444 is quite likely. And it can be tested more easily than most of the other ideas.

 

[Herman Trivilino]

I fill the bag to almost cover the clay, but not completely. Now that this could be part of the issue, I realize I've never asked if any potters cover the clay with water. I'm pretty sure local "lore" is to partially fill the bag.

As @jbriggs444 and others have noted I think this is the key.

The only thing I would add is a comment about the significance of the pressure from submerging the bag. It may seem a small pressure but that is a relative term. CPAP pressures, for example, are on the order of 10 cm of water. So submerging the bag to a depth of, say, 30 cm or more creates a large pressure by comparison. The highest CPAP pressure is 20 cm, or 25 cm for some CPAP machines.

 

[256bits]

I see there are also chemistry forums here. Is there a way to cross post or maybe I should copy this post and put it in the chem sections? Or tag some people there and ask them to look here? I don't know how much of a crossover there is between sections.

I would not recommend cross post.
Most users can and do scan threads and will add relevant information as they deem fit.
Cross post will destroy continuity.

 

[ImaginaryTango]

I guess I took the diagram too literally then. In this case I agree with @Dale that the mechanism proposed by @jbriggs444 is quite likely. And it can be tested more easily than most of the other ideas.

Yes, and I will be testing it. However, due to my clay stock just about running out so I'll be getting more fresh clay, it'll be awhile before I can test it. I'm sure the thread will have petered out by then, but I'll come back and report what I find.

The only thing I would add is a comment about the significance of the pressure from submerging the bag. It may seem a small pressure but that is a relative term. CPAP pressures, for example, are on the order of 10 cm of water. So submerging the bag to a depth of, say, 30 cm or more creates a large pressure by comparison. The highest CPAP pressure is 20 cm, or 25 cm for some CPAP machines.

I've been thinking about that. I took scuba lessons decades ago and remember that every 33', in salt water, pressure increases by 1 ATM - but the human body does not compress, since it's mostly water. Clay does not have as high a percentage of water as mammals do, but I've seen a piece lose 33% or more of its weight after the bisque firing. That (the way I do it) is a firing that heats up to 180°F and sits there for hours to make the loose water evaporate. Then it ramps up to get to the point of inversion, where the water that's bonded to silicon in the clay is released and the receptors in the silicon molecules are blocked or inverted (I'm not clear on just what happens), so the silicon won't bond with water anymore.

I've seen it lose more weight in the glaze firing, so I'd say it's over 1/3 water. I'm thinking there might be some kind of balance in there that may just need a small amount of pressure to make the water infuse all the way in.

I would not recommend cross post.
Most users can and do scan threads and will add relevant information as they deem fit.
Cross post will destroy continuity.

Okay, thanks. Won't worry about it then.

 

[sophiecentaur]

Is there something wrong with this thread / page? I can only see this page of posts. When I try to go back to the first page, I'm stuck on an image of experiments (thought or real) with a block of clay in different situations. I'd like to see the early posts but I can only see the latter half.
The experimental details could be very relevant.
Cheers

 

[renormalize]

Is there something wrong with this thread / page? I can only see this page of posts. When I try to go back to the first page, I'm stuck on an image of experiments (thought or real) with a block of clay in different situations.

I have no trouble going back to the first thread page, where post #1 does indeed show various situations for soaking clay. As far as I can see, no other posts in the entire thread have similar pictures. What is the lowest post number that you can view?

 

[Herman Trivilino]

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.

That explanation seems to work equally well for scenarios B and D. In other words, why the bucket?

 

[sophiecentaur]

I have no trouble going back to the first thread page, where post #1 does indeed show various situations for soaking clay. As far as I can see, no other posts in the entire thread have similar pictures. What is the lowest post number that you can view?

Hi.
This is weird. Entering the thread, I am taken to post #31 (start of page 2.) When I click on the page 1 option, I just get that figure with no way out, into the thread. Never had that happen before and others have managed to carry on the conversation and, presumably, get to the first page. I could try another browser and that may sort it.

 

[sophiecentaur]

I have no trouble going back to the first thread page, where post #1 does indeed show various situations for soaking clay. As far as I can see, no other posts in the entire thread have similar pictures. What is the lowest post number that you can view?

Sorted. Finger trouble on my part. I was faffing about, trying to see my WiFi manager pages and had tried altering some browser settings. I had altered the 'Reader' settings.
Cheers.

 

[sophiecentaur]

There is one difference between D and the others and that could be the air pressure on the surface of the water in the bag. Very little difference, of course but it may alter the absorbed CO2 (?) levels in the water in the bag. Could that affect the progress of water in the clay? Some subtle change at the gaps between clay particles, perhaps. I may have missed this idea if it appeared higher up but I didn't look scrupulously through the previous 40+ posts.
If the experiment were repeated with boiled or distilled water and carbonated (fizzy) water, there might be a difference.

 

[256bits]

If the experiment were repeated with boiled or distilled water and carbonated (fizzy) water, there might be a difference.

Lots of testing coming in for a scientific discovery and verification/

 

[sophiecentaur]

Lots of testing coming in for a scientific discovery and verification/

To be honest, clay has been used for thousands of years and the best methods will have been established for almost that long. The Science may be interesting (= IS interesting!) but we should look for established best practice first ( no experiments needed) and the Science used to explain that. I wonder what additives are used by the Big Boys in pottery. Time is money for them and I bet they know why.

See also Concrete methods.

 

[ImaginaryTango]

There is one difference between D and the others and that could be the air pressure on the surface of the water in the bag. Very little difference, of course but it may alter the absorbed CO2 (?) levels in the water in the bag. Could that affect the progress of water in the clay? Some subtle change at the gaps between clay particles, perhaps. I may have missed this idea if it appeared higher up but I didn't look scrupulously through the previous 40+ posts.

I am betting that whatever the reason, that the "very little difference" point is going to be part of it. Maybe just a slight increase in pressure, or or just a slight difference in gases or particles in the water due to the plastic acting like a permeable membrane. I would think it's a small change that's "just enough" to make a difference.

Lots of testing coming in for a scientific discovery and verification/

And not much chance it would lead to a Nobel Prize, either...

To be honest, clay has been used for thousands of years and the best methods will have been established for almost that long. The Science may be interesting (= IS interesting!) but we should look for established best practice first ( no experiments needed) and the Science used to explain that. I wonder what additives are used by the Big Boys in pottery. Time is money for them and I bet they know why.

I don't know how long this method has been in use, but I'm betting since clay started being shipped in plastic bags and that someone found it just by testing it and finding it worked.

As to the "Big Boys," that's likely irrelevant for several reasons. Clay, generally, as shipped, is saturated enough and stays that way until the bag is opened. If it's a big company and they're throwing it or using it for slab work, it's going to be used in large amounts without much leftover. Most pottery you see in stores is made with slip casting, and slip is about the consistency of a milkshake, so that's different. (There may be more situations that I'm not aware of.) Regarding additives, well, a lot of potters are going to use their own stuff and might be doing more to their clay for a specific result they want, but I think big companies are going to be using a lot of different methods for mass production than what individual potters use.

So I'd consider this method the tried and tested one most potters use.

 

[sophiecentaur]

So I'd consider this method.

Do you mean D? (just to be sure)

There have been 'Big Boys' for a long time and experts may well have needed to store clay for a while. I'd bet your problem has been solved by someone else before you.

 

[A.T.]

That explanation seems to work equally well for scenarios B and D. In other words, why the bucket?

In D you have more pressure than in B. I would also not take the diagrams too literally, especially the part how much of the clay is submerged in the bag. This might differ between B & D.

 

[ImaginaryTango]

Do you mean D? (just to be sure)

Yes, D.

There have been 'Big Boys' for a long time and experts may well have needed to store clay for a while. I'd bet your problem has been solved by someone else before you.

The problem isn't what to do or how to do it. This is what works and what potters do. So refreshing the clay has been solved. The question is WHY does it work?

In D you have more pressure than in B. I would also not take the diagrams too literally, especially the part how much of the clay is submerged in the bag. This might differ between B & D.

We have talked about that, but I thought the "more pressure" was dismissed. If it's still considered an answer, my question (and I may have missed this) is how do we know there is more pressure (in D than in B or C) and just how is it created?

 

[tech99]

I have tried to read all the replies, but no one seems to have suggested Osmosis. When the clay is placed in bag of water, salts from the clay will dissolve in the water in the bag and form a solution. As this solution is more concentrated than the water in the bucket, water will pass through the plastic film by Osmosis and permeate the clay.

 

[Herman Trivilino]

In D you have more pressure than in B.

I understand but I don't see the role it plays in process described by @jbriggs444. That's what I was responding to.

 

[sophiecentaur]

In D you have more pressure than in B. I

Yes but not greater than C, where the heads of water are the same (in the diagram - the added water would have been adjusted) so the only difference would be due to a. the presence of air under pressure or b. the plastic membrane.

no one seems to have suggested Osmosis.

Significant Osmosis is through semi permeable membranes but polythene bags don't leak, except to the subtle extent of affecting the crispness of potato crisps / chips. Crisp packets are not made of regular plastic. But, in any case, any added volume of water in the bag would not affect the static pressure as long as the bag isn't stretched.
It's true that added pressure is used to enhance soaking of timber with preservative but just using deeper water can improve penetration into the clay, here.

 

[A.T.]

Yes but not greater than C

The reason for D working better than B doesn't have to be the same as the reason for D working better than C.

 

[A.T.]

I understand but I don't see the role it plays in process described by @jbriggs444.

The vertical pressure gradient in the medium surrounding the bag, is greater in D than in B. This affects the pressure gradient in the bag, which squeezes out the air upwards, out of the clay.

 

[ImaginaryTango]

The vertical pressure gradient in the medium surrounding the bag, is greater in D than in B. This affects the pressure gradient in the bag, which squeezes out the air upwards, out of the clay.

That works with my earlier comment that people keep talking about small pressure differences and some things have been ruled out because of that. But I think a small difference may be just enough.The pressure gradient would not be huge, but it could still be just enough.So what you're saying (I'm doing my rephrasing thing, including questions, to be sure I follow you!), is that when it's not in the bag, the water comes in from all angles, including the top, and there's not enough pressure to push the air out through that 1" or so that has infused. But with a pressure gradient, in D, it helps push the air up through the top, with enough energy to push it through any barrier that top 1" of clay might provide with the water in it?I'm thinking one thing I can do to test this would be to pull clay out after a few hours and see if the bottom is infused more than the top (except that top 1").

 

[Herman Trivilino]

But I think a small difference may be just enough. The pressure gradient would not be huge, but it could still be just enough.

Just checking to make sure you understand what is meant by pressure gradient as opposed to pressure. Pressure gradient refers to the change in pressure with position. So in this case as you move upward from the bottom of the bag of clay to the top, a distance of just a few centimeters, the pressure decreases. I agree that this is a small difference, but it is actually what's responsible for buoyancy, which is the upward force exerted on a submerged object by the surrounding fluid. So it is large enough to produce at least that consequence.

I'm glad that you got more help at PF in providing explanations, and that you've received some information about possible experiments you could perform to test those explanations. I really doubt this is a chemistry thing, it really seems like a physical phenomenon to me.

 

[ImaginaryTango]

Just checking to make sure you understand what is meant by pressure gradient as opposed to pressure. Pressure gradient refers to the change in pressure with position. So in this case as you move upward from the bottom of the bag of clay to the top, a distance of just a few centimeters, the pressure decreases. I agree that this is a small difference, but it is actually what's responsible for buoyancy, which is the upward force exerted on a submerged object by the surrounding fluid. So it is large enough to produce at least that consequence.

Yes, and thank you for verifying that. While I haven't done it in decades, at one time I had scuba experience. But when you think of 33' as 1ATM, it's easy to think of 1' as not a big deal, so until someone pointed out it's small, I don't think I thought of that as an issue. It's fascinating to me that even with that 1' there's a gradient and that such a small amount could still be enough.

I'm glad that you got more help at PF in providing explanations, and that you've received some information about possible experiments you could perform to test those explanations. I really doubt this is a chemistry thing, it really seems like a physical phenomenon to me.

I really appreciate the help I'm getting here. I used to teach and have seen teachers that get fed up with students who don't pick something up immediately and others who can give fantastic explanations - and others who realize that what works for one person doesn't help another. I feel like I'm learning and picking things up here and it's great that people are willing to help someone with limited experience!

Like you, I don't think it's a chemistry thing. I have a good friend who was a Physic major when we met, in college, and his first comment was, "I think it's a chemistry thing." (I think he was thinking, "I don't see what's happening here, so it must not be physics." I'm not sure, so I'm not trying to say that as a criticism.)

 

[sophiecentaur]

The vertical pressure gradient in the medium surrounding the bag, is greater in D than in B. This affects the pressure gradient in the bag, which squeezes out the air upwards, out of the clay.

I can see that the pressure at a given height above the floor is greater in D than in B (a different head of water) but why is the gradient different? The pressure in the air in the bag will be equal to the pressure difference between the two water levels. (Won't it?) That increase in air pressure at the surface will cause more absorption of air into the clay water. This my original point. Solubility will vary for the different air components; no appreciable difference for O and N but CO2, perhaps not. But how much CO2 will there be in the air in the bag.

In all this we're presumably assuming a flexible bag which is more or less slack at the top. Perhaps a cylinder with freely moving pistons at the top and bottom would be easier to discuss. Not very convenient to engineer, though - and the water level must be adjusted during assembly.

But when you think of 33' as 1ATM,

A few inches can make a huge difference to how easy it is to breathe from a demand valve as your body tilts away from horizontal. (My DV was a bit ancient so things may have changed but your diaphragm notices these things - same as breathing whilst floating at your back and leave the horizontal position. (on the surface).

 

[A.T.]

The vertical pressure gradient in the medium surrounding the bag, is greater in D than in B.

why is the gradient different?

Because the density of water is greater than the density of air.