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Clay thickness, in relation to physical strength

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So I make some relatively thin wares (approx. 1/8"), and a lot of my customers are surprised at the light weight of the pot, but then ask if the durability is the same. I have tested my pots in the microwave, oven, freezer, and dishwasher and find them to handle the stresses of thermal shock well, and have no concerns in this regard. My pots also get packed lightly when traveling to shows, in some cases, no packing between them, just kept from bouncing around. Ive had a few pots (out of tens of thousands over the years) get broken during transport, but find that my work is quite durable in relation to its ability to handle mechanical shock. So, my response to my customers, is that given normal care/handling, that they are every bit as durable as thicker pots. Yes, thicker pots will hold heat differently, and there are pots which I like to feel heavier, but in a purely mechanical sense, I personally dont find the weight gain, to be beneficial to the strength of my wares.

My question though is more of the physical strength of the clay itself (no glazes in question here); has there been any investigations into clay wall thickness in regards to mechanical strength? i.e. if a clay wall of 1/4" thickness handles 10# (made up number; just for discussions sake) of force, does doubling the wall thickness to 1/2" increase its strength to 20#. Is the growth linear? Exponential? I understand that clay handles different forces better than others; great in compression, but poor in tension, so not really interested in comparing those stresses against each other.

Im aware that different clay bodies, have their certain strengths, during regular use, compared to others,  but different clay bodies aside, is there a major growth in strength of the ware with increasing the wall thickness?

Could a 1/2" thick handle, deal with the stress of being banged into a cupboard, more than a 1/4" handle? Yes, my pot falling to the tile floor is going to die, and likely a 1/4"-1/2" pot will too, and while a 1" thick wall may not(pity the tile floor in this case), its obviously not going to be fun to use because of the weight. So this is more of a discussion of technicalities, not practicality.

If there have been studies done on this, would you share the data, or a link to the data (if online). I would imagine that this is something that is learned while becoming a ceramic engineer, and that industry is aware of the physical limitations of their objects, but for us potters, Ive never come across data relating to this. Its always been "that pots should be between 1/4-3/8" thick", but is this just because its a relatively easily attained thickness for beginner potters?

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I make everything pretty thin too, my thoughts are that people treat light thin things more carefully than they do thick things that feel durable.  My mugs are usually between 1/8th and 1/16th of an inch thick and survive a fall onto gravel from 4 feet pretty well, don't ask.

 

I'm sure it gets stronger as it gets thicker, but there's that balance of strength, and weight and visual appeal

Edited by liambesaw

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Sam,

I think your question is: "how does the strength of a fired ceramic piece correlate with the thickness of the piece"?  

My thoughts are: 
Pete Pinnell sometime back published an article on the breaking strengths of various fired clay bars; the experiments were part of a class assignment.  Check the back issues of Clay Times Magazine.  My memory of the was that The experimental results were interestingly presented, but seemed inconsistent (to me and a few others) with standard materials theory.

The answer is more likely be found in the literature of material science of composite solids than in the studio pottery literature.

If I were to do a study, I would first become familiar with the strength of materials segment of mechanical engineering (relabeled as Materials Science in some engineering curricula.  Cured concrete is a probable analogy to fired ceramics for making relative inferences. 

Another starting point would be to study the strength of commercial wall and floor tiles.  The American Ceramics Society journals and conference proceedings would be a good starting point.  The World Congress of Ceramic Tile Quality conferences proceedings, and the publications of organizations supporting the congress, could also be a reliable source of insight into the strength of you fired pots.

A separate line of inquiry you should become familiar with is the methodology of formal strength testing of brittle materials and how the measured results are used in making design decisions.  My experience has been that the value of a single test piece is insufficient for making reliable decisions.  We needed several hundred tensile strength data points before a usable average and 1 sigma error band emerged from the test on new material formula for composite material.  

From my experience in the polymer composites materials the particle size of fillers and the specimen dimensions were important variables.  By analogy to fired ceramics, I would expect that the minimum thickness of a mug wall would also be constrained by the largest size of the particles in the clay body matrix.  Coarse grog would most likely be one of the constraining ingredients, as would be the porosity of the fired ware. 


As a final tidbit:  The ACerS annual MS&T conference each year there is a student "Mug Drop" contest.  You might chat with some of the students that have participated. 


LT

P.S.  I am still reassembling the reading list of science references for ceramics.  hope to be done this weekend.

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1 hour ago, liambesaw said:

I'm sure it gets stronger as it gets thicker, but there's that balance of strength, and weight and visual appeal

Undoubtedly; assuredly I could make a mug, with thick enough components to withstand a fall off a ladder onto concrete. Could one use it with enjoyment....nah. As one might say, thick in the right places!

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1 hour ago, Magnolia Mud Research said:

I think your question is: "how does the strength of a fired ceramic piece correlate with the thickness of the piece"?

Correct! Obviously I was trying to say that in my OP, and thankfully I made enough sense that you could simplify it into a one sentence question.

Yes, I am just curious to the strength of fired ceramics, in numerous thicknesses, and shapes, put under different stresses (shear vs tensile vs etc), and how thickness impacts strength.

1 hour ago, Magnolia Mud Research said:

The answer is more likely be found in the literature of material science of composite solids than in the studio pottery literature.

I kind of was assuming this; Ive read a lot of different studio related publications and have never come across anything like what I was imagining. Not having an engineers bone in my whole body, I knew that somewhere out there, there would be information related what to I am intrigued by, but would have had no idea on where to begin.

I appreciate the info you provided; lots of good places to start a query. This is more of a idle mind query, so no guarantees that I can report back to this post with conclusive evidence in a short time frame, but I do hope to learn a little more about it.

It would be fun to conduct some tests on my own, and maybe at some point I will have some time to devote to doing this. It would be interesting to see how the results could impact utilitarian pottery in regards to optimal design for structural integrity. If no data like this is widely accessible, it would be interesting to know what we could be doing differently. i.e. does a handle which has an attachment point of 1 square inch have 50% more resistance to being pulled from the vessel than one that has .75 square inches? Of course, like you mentioned, this information would not be applicable to all clay bodies, as the formulation of the clay body itself imparts different strengths, and as well, the design of the vessel and the construction methods.

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Hitch:

the term you need to research is MOR (modulus of rupture) as LT pointed out, particle size is a major player in green and fired strength. For instance in green strength: a 0.75 micron ball clay hovers around 400 psi. A 0.30 micron ball clay can approach nearly 800 psi. Particle size distribution is a common theory in clay formulation: but packing density increases psi.  Large particle/ mesh weakens a body because of the porosity. Glass/mullite is stronger, increasing the content allows for thinner walls. Here is a place to start

https://books.google.com/books?id=vtzkmgkvYj8C&pg=PA10&lpg=PA10&dq=modulus+of+rupture+for+clay+bodies&source=bl&ots=SWcZDpO-rL&sig=ACfU3U2UxN8hNvngWGeLi5hFKByZ9RvZ3A&hl=en&sa=X&ved=2ahUKEwi40cTx2JbgAhUE94MKHRwYCiUQ6AEwEnoECAkQAQ#v=onepage&q=modulus of rupture for clay bodies&f=false

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15 minutes ago, glazenerd said:

a 0.75 micron ball clay hovers around 400 psi. A 0.30 micron ball clay can approach nearly 800 psi

See, to me that is awesome info; a particle size less than half of its counterpart has double the psi. I appreciate the link....down the rabbit hole I go!

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Coming out of Collage I made potato chip pots(1976). The goal was thin. Over time my customers told me they where breaking them. Within 10-15 years I started to make them thicker-this was  mostly stoneware  then. Mid eighties I started making 50/50 porcelain and stoneware funtional wares. The stoneware chipped much easier and sold slower . I switched to all porcelain within a few more years I started to get it about thickness. Someone brought me a bowl in the early 2000s I made that was dated 1980-it was  super thin large bowl -it was broken and they wanted another-Not so thin I said now and they had no issues with that.

I think for everyday use a this pot has limited lifetime as a super thin pot. A clunky pot also is a bad idea. Nobody likes a clunker .The right thickness is just right.

The other factor in the weight thing is age. Many potters do not think about this subject.

I made a set for my mother who later in life said its just to heavy-It passed on to my older sister when out mother died (1987) later in life she decided it was to heavy and wrong color. I took it back in trade for a porcealin set ,color of her choice and made a we bit thinner-now she says some years later to heavy.Older folks just loose strength and most hand made ceramics are just to heavy. something also to consider .Your customers age group.

All the strenght /micron talk comes down to the customers/humans thought process of this feels right for the form

 

Edited by Mark C.

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From a strength and materials aspect the materials are quite variable and the geometry often complex. All these things could be analyzed and without question ceramics is similar to concrete in that it is very good in compression but not very good in tension and or bending. So without complicating things too much obviously paper thin is certainly fragile in bending and impact.

My practical opinion without resolving this to a modulous of elasticity or allowable stress in a material or developed moment of inertia  of a shape or shear and moment stress analysis on handle connections would be: shapes  thrown uniformly in geometry with uniform reasonable thickness provide the maximum reasonable and practical strength given the material used. Calculating this for hand thrown objects .......very difficult.

Porcelain will probably be more resistant to chipping than stoneware. Thick sections will withstand more direct impact than thin.

And the most often overlooked, a glaze fit that keeps your clay in slight compression adds greatly to the final overall strength of the ware. This is relevant when we teach folks as you adjust your glaze from severe crazing to minor to non crazed often provides a good point where your glaze is actually strengthening your pot by as much as 20% or as much as 95% more than a crazed glaze.

well thrown uniform geometry uniform in thickness using homogeneous materials with a slightly compressive glaze with respect to the claybody seems to be the answer. In real life in my opinion, we can calculate all of this on a one off basis but I suspect  that after a lifetime of throwing most potters settle on their quality based on their throwing preferences, claybodies and time tested attachments.

and yet I made no mention of thermal stress. Yikes, this is complicated. I think in most cases experience counts a bunch for handmade products.

Edited by Bill Kielb

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30 minutes ago, Bill Kielb said:

This is relevant when we teach folks as you adjust your glaze from severe crazing to minor to non crazed often provides a good point where your glaze is actually strengthening your pot by as much as 20% or as much as 95% more than a crazed glaze.

I've seen mentioned, recently on digitalfire, that a crazed pot is 1/3rd the strength of an unglazed pot, and 1/6th as strong as a pot with a glaze that fits well.  Amazing to me, but thinking of each craze line as a perforated "tear here" line makes sense.  You can feel it on a crazed pot just from the sound it makes.

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@liambesaw

An additional helpful view of this from the compression standpoint: if we can keep concrete in compression, suddenly it can flex like a diving board (extreme example). Prestressed concrete is created on this basis and allows thinner sections to develop more ultimate bending strength. .

There  are many examples of prestressed flexible concrete on you tube today which still amaze me that the thing stays together because the cables or tension members are stretched so tight that the concrete cannot go into tension.

A nice glaze squeezing your pot ever so slightly really increases the strength In much the same way. Although I really like your example of perforations in the case of the crazed glaze for its clarity and simplicity and every study I have seen reflects that this representation is very accurate.

Since I have been around prestressed stuff, the other side of this makes total sense to me as well and may help others visualize why slight compression exerted by the glaze can add considerable strength to the finished ware.

 

Edited by Bill Kielb

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Mark:

nothing like the voice of experience.

the third parameter is material access. The various clays sold routinely in the pottery trade represents less than 10% of the clays actually available. I buy directly from Old Hickory mines in KY: of the nearly 30 varieties they mine, only two are sold in this trade.  Manufacturers have custom mixes blended at the mine, or custom slurries shipped in on rail cars. All the science and chemistry has been done before they unload product. When you stop and think about it: potters have an extensive database rolling around in their heads at any given time. Material science, chemistry, production, equipment use and repair, marketing, book keeping, and shipping. Impressive actually.

Tom

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7 hours ago, Bill Kielb said:

if we can keep concrete in compression, suddenly it can flex like a diving board

I learned about concrete being under compression in bridge construction (not in the biz, just on a science channel documentary). Huge cables, put under god knows how much tension, keeps the concrete under compression, and allows us to build much leaner, and much stronger bridges. It is amazing to see concrete flex like that!

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9 hours ago, Bill Kielb said:

My practical opinion without resolving this to a modulous of elasticity or allowable stress in a material or developed moment of inertia  of a shape or shear and moment stress analysis on handle connections would be: shapes  thrown uniformly in geometry with uniform reasonable thickness provide the maximum reasonable and practical strength given the material used. Calculating this for hand thrown objects .......very difficult

This is the kind of info I am wanting to learn more about. Understandably, in a lab setting, with strictly controlled tolerances the physics would make a huge impact, and the material could be understood more. In hand thrown objects, made in a studio setting, there are just WAY too many variables that could impact the final strength of the object, and without knowing which of the numerous variables was the cause, it would be a cat and mouse game.

I agree; make wares that are uniform in their wall thickness, with a well fitting glaze is the key. In the practicality of this query I am settled on my process, but in the technicality of the idea, just curious.

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14 hours ago, Bill Kielb said:

@liambesaw

An additional helpful view of this from the compression standpoint: if we can keep concrete in compression, suddenly it can flex like a diving board (extreme example). Prestressed concrete is created on this basis and allows thinner sections to develop more ultimate bending strength. .

There  are many examples of prestressed flexible concrete on you tube today which still amaze me that the thing stays together because the cables or tension members are stretched so tight that the concrete cannot go into tension.

A nice glaze squeezing your pot ever so slightly really increases the strength In much the same way.

A non-crazed glaze is (probably) not under tension, and may be under slight compression. But if the glaze is under compression because the clay-body contracted more than the glaze, wouldn't the clay-body be under tension? I've also heard the claim that glazes under slight compression strengthen ware, and while I don't doubt that it's true, I can't figure out why this is.

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@Pieter Mostert

The way I keep it straight in my head:

If the glaze crazes it is because the glaze shrinks faster (more) than the  Clay. The glaze is in tension but the tension exceeds the strength of the glaze so it cracks. If the glaze shrinks very very slightly faster (more) than the clay, the clay is in compression, glaze in slight tension but the tension does not exceed the strength of the glaze.  It now becomes the tension member in our prestressed concrete or a rubber band around our ware.

when glazes go in compression the opposite is happening and will shiver off the pot when compressed enough. A good way I envision this is a buckle in the pavement. On a hot summer day the surface expands so much it puts itself into compression and since it is constrained eventually exceeds the modulus of rupture of the material and it erupts or buckles upward where not constrained.

to answer your last question, actual tests verify that claybodies in slight compression are up to 20% stronger.

Edited by Bill Kielb

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On 1/30/2019 at 2:27 PM, Magnolia Mud Research said:

Pete Pinnell sometime back published an article on the breaking strengths of various fired clay bars; the experiments were part of a class assignment.  Check the back issues of Clay Times Magazine.  My memory of the was that The experimental results were interestingly presented, but seemed inconsistent (to me and a few others) with standard materials theory.

About 1/2 way down this link Pete Pinnell discusses his results from his MOR testing. 

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  On ‎1‎/‎30‎/‎2019 at 4:27 PM, Magnolia Mud Research said:

Pete Pinnell sometime back published an article on the breaking strengths of various fired clay bars; the experiments were part of a class assignment.  Check the back issues of Clay Times Magazine.  My memory of the was that The experimental results were interestingly presented, but seemed inconsistent (to me and a few others) with standard materials theory.

Agreed - his modulus of rupture is a good indicator in bending but defining a wares ultimate strength I suspect a bit more complicated. I had an interesting discussion once with someone who declared stoneware was strongest! I asked, how do you define strength and they immediately seemed miffed and dismissive. I clarified: Tension, compression, flexural, hardness? They rolled their eyes and said everyone knows stoneware is strongest!  So there you have it.

So after reading Pete's post, I find his results interesting. As to porosity and the efficacy of vapor glazes, (Linda - I believe) not so much. I think Ron's questions were spot on and am always concerned with generalities that Linda and Pete presented but I find her work interesting and Pete's work as well.

I am not enthused about the Cristobalite reference  and find having it is unlikely, unless fired to cone 14 and free silica, but OK, sort of not sure its worth putting out there. I believe many folks like to go slow (whatever that is?) through quartz inversion but once you ask them what is slow or since quartz inversion happens at a specific temperature and your pot top to bottom certainly is not the same temperature they tend to back off the position or become dismissive.

Vapor glazes and food safety not on the top of my list for compatibility and absorption has several food safety issues about it, including issues from boiling (thanks microwave) to freezing.

So all in all interesting and valid in some ways but in others  maybe not ………………….. except for glazes that compressed the clay body increased their bending strength!

Just my opinion reading this quickly.

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Interesting read Min, TY. Although he is referencing MOR values alone: and cone 04 produced the highest values. Two things missing in that observation: 60% red art delivers a fair amount of iron disulfide. I wonder how much the iron reduced, and accelerated the density of that perticular body?  I realize PSI was the topic, but absorption is just as important.  Linda discussed cristabolite inversion: a topic rarely addressed in oven ware. One of my favorites was the research done by Ougland & Brindley of The British Ceramic Society. They used x-ray defraction to quanitify their studies. The amount of glass, mullite, and free silica in a porcelain body at C6 and C10. 

2192F (1200C.). Glass 62.   Silica 21.    Mullite 19
2372F (1300C).  Glass 66.   Silica 16.    Mullite 21

glass content is good, but mullite is the tough stuff that gives a body strength. Mullite can be manipulated by the Si:AL ratio to some degree.

T

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22 hours ago, Bill Kielb said:

@Pieter Mostert

The way I keep it straight in my head:

If the glaze crazes it is because the glaze shrinks faster (more) than the  Clay. The glaze is in tension but the tension exceeds the strength of the glaze so it cracks. If the glaze shrinks very very slightly faster (more) than the clay, the clay is in compression, glaze in slight tension but the tension does not exceed the strength of the glaze.  It now becomes the tension member in our prestressed concrete or a rubber band around our ware.

when glazes go in compression the opposite is happening and will shiver off the pot when compressed enough. A good way I envision this is a buckle in the pavement. On a hot summer day the surface expands so much it puts itself into compression and since it is constrained eventually exceeds the modulus of rupture of the material and it erupts or buckles upward where not constrained.

to answer your last question, actual tests verify that claybodies in slight compression are up to 20% stronger.

Are you sure the tests didn't involve glazes in slight compression? I would think that, given the relative thicknesses of glaze and claybody, the compressive force exerted by a glaze on a claybody would be minimal.

I did a bit of digging, and came across this Alfred Masters thesis by Jennifer Benson. After skimming through this, it's still 100% clear to me why an increase in the compression of a glaze should increase the strength of the ware (up to a point), but I think it may be due to the fact that this reduces the likelihood of cracks originating in the glaze.

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The thesis might be a good introduction to the detailed thinking that is required to fully understand the 'science' associated with the simple question originally asked by Sam H.  The bibliography attached to the thesis is a reading list for additional background on the main topic.   Thanks, Pieter for including the link to the thesis. 

The thesis also, in the final paragraphs of the conclusions, makes the point that adding silica to a glaze may cause an increase in thermal expansion. quote: 
"Undissolved quartz was present in Glaze B.  The quartz caused an increase in the thermal expansion of the glaze.  When the glaze was fired to a higher temperature and all of the quartz was dissolved, the thermal expansion of the glaze decreased.  Therefore, it is not always true to say that increasing the amount of silica in a glaze will lower its thermal expansion.  This is only true if there is no undissolved quatz (sic) present in the glaze" [Benson, Jennifer L. "Effects of Glaze Variables on the Mechanical Strength of Whitewares." (2015). pg. 67] 

LT

 

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4 hours ago, Pieter Mostert said:

Are you sure the tests didn't involve glazes in slight compression? I would think that, given the relative thicknesses of glaze and claybody, the compressive force exerted by a glaze on a claybody would be minimal.

I did a bit of digging, and came across this Alfred Masters thesis by Jennifer Benson. After skimming through this, it's still 100% clear to me why an increase in the compression of a glaze should increase the strength of the ware (up to a point), but I think it may be due to the fact that this reduces the likelihood of cracks originating in the glaze.

Maybe but all the tests I have reviewed indicate wares in compression are stronger. Tony Hansen was mentioned earlier and I have seen Matt Katz’s data as well  with respect to two decent sources for this information.

If true it would be consistent with other things we find  which is why the discussion of prestressed concrete was presented. Certainly wares with glazes that craze are weaker so I am pretty happy with this as reasonably accurate.

just my opinion though

Edited by Bill Kielb

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I don't know if this applies at all to ceramics, but I'll put it here and you all can figure out if there are any connections:

I spent 3 years working for a residential/commercial glass shop in Ames, IA when I got out of grad school, while my wife finished up vet school at Iowa State. In addition to installing store fronts and putting mirrors in every new house in town, I was in charge of all the cut work in the shop. I spent at least half of every day cutting glass, everything from single strength (1/16" thick), double strength (1/8") and plate (1/4"), to laminated and wire glass. Stock sheets were as small as 36"x72" and as large as 96"x 144". Every now and then I'd get to cut some 3/8" and 1/2" thick, which was fun stuff to work with. Anyway, when cutting glass, the pressure applied while scoring is critical. Intuition says the thick glass would require more pressure than thin glass to get a good score, but the opposite is true. More pressure is required to get a good score on 1/16" than 3/8" thick glass. Press too lightly on thin glass, and the cut won't run straight. Cut too hard on thick glass and the cut will flare badly.

Again, I don't know how, if at all, this relates to ceramic bodies and/or glazes and crazing, but I'll let you all figure that out. Have fun!

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Thanks for posting that link Pieter, interesting read. 

@Magnolia Mud Research, what comes to mind when I read about undissolved quartz and the coe in Pieter's link was a silica progression blend done by Michael Bailey demonstrating the effect of undissolved silica in a glaze. (Glazes Cone 6 - Michael Bailey 2001 pages 59-61) Screen shot below of the base recipe on the left and on the right is the same recipe with an increase of 10 grams of flint. I can't post the pictures here (copyrights) but the increased flint content caused major crazing of the glaze.  As the progression blend increased from recipe 1, which demonstrated zero crazing and well melted gloss glaze, up to a huge 50 grams of added flint content the crazing went up in relation to the added silica the glaze wasn't able to melt. With glaze calc recipe B still falls well within the "limits" of a ^6 glaze and the COE is lower than in original recipe but there is obviously undissolved silica present as witnessed by the now matte and crazed quality of the glaze. (The other interesting thing is there is no boron in this cone 6 glaze.)

642518969_ScreenShot2019-02-01at4_49_05PM.png.a3348f0325db4d718ad9dc9d2010c230.png

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