Increase the Strength of Low Fire Clay | Aggregates

[luca 0]

Hello all,

I am engaged in a product design venture involving ceramics in an unusual application and must seek out additional expertise of experienced ceramists. Though I'm unable to fully describe the product, I can provide a sense what properties are sought from the ceramic body in order for it to perform its function well, which I'd be very interested in hearing others comment on.

 

To begin with, the fired piece is intended not to ring or resonate such as an instrument would when struck. We've experimented with various clay bodies for this including porcelain, stoneware, and earthenware, experimenting with a range of firing temperatures for each as well as with thicker and thinner walls. The ideal clay body for the application seems to be the low-fire type of clays which do not vitrify, and therefore do not ring in a way that is characteristic of porcelains, and stonewares. Our first concern with earthenware, however, is it's lower strength as compared to other ceramics. I'm aware that the use of aggregates in the clay body ("grog") can strengthen the piece, but have no experience in this matter. It is the first area that I'd hope to gather information on through the forum.

 

What types of aggregates or additives have others here used to successfully increase the strength of slip cast earthenware? I'm keen to hear any tips, no matter how unusual or uncommon, on developing better strength in low-fire clay. Our finished pieces are expected to remain unglazed.

 

All responses appreciated - thank you!

Luca

[OffCenter 84]

Hello all,

I am engaged in a product design venture involving ceramics in an unusual application and must seek out additional expertise of experienced ceramists. Though I'm unable to fully describe the product, I can provide a sense what properties are sought from the ceramic body in order for it to perform its function well, which I'd be very interested in hearing others comment on.

 

To begin with, the fired piece is intended not to ring or resonate such as an instrument would when struck. We've experimented with various clay bodies for this including porcelain, stoneware, and earthenware, experimenting with a range of firing temperatures for each as well as with thicker and thinner walls. The ideal clay body for the application seems to be the low-fire type of clays which do not vitrify, and therefore do not ring in a way that is characteristic of porcelains, and stonewares. Our first concern with earthenware, however, is it's lower strength as compared to other ceramics. I'm aware that the use of aggregates in the clay body ("grog") can strengthen the piece, but have no experience in this matter. It is the first area that I'd hope to gather information on through the forum.

 

What types of aggregates or additives have others here used to successfully increase the strength of slip cast earthenware? I'm keen to hear any tips, no matter how unusual or uncommon, on developing better strength in low-fire clay. Our finished pieces are expected to remain unglazed.

 

All responses appreciated - thank you!

Luca

 

 

Luca, your first assumption is incorrect. Grog weakens the clay body. Grog gives clay strength during throwing and has other benefits but actually weakens the fired clay but only a little and depending on how much grog is used. A year or so ago a well-known potter's, Pete Pinnell, college class tested a lot of clays for strength and his test and others proved that clay strength depends on the clay reaching full maturity, not how high it was fired and Pete's tests even showed that earthenware was stronger than any of the other clays (cone 6, cone 10, etc.) tested. There were some who disputed the earthenware part of the test's accuracy later because of a thickness issue. But, my point here is that grog and other aggregates weaken clay so if you want to make your clay stronger get rid of the grog and other aggregates and fire to full maturity of the clay. Do not depend on what the clay maker claims. You have to test. While you're testing to see where the maturity point is, you could also test other earthenware clays to find a stronger one.

 

Jim

[luca 0]

Thanks Jim - That's very useful to hear, and would indeed set me down along a different path. With full maturity, just to be clear, this means firing right to a clay's recommended temperature, and not at all over or under, yes? It's really very surprising that Pinnell's tests showed earthenware to come out so strong, almost flying in the face of all assumptions about stoneware strength vs earthenware's, even if this was a function of wall thickness. For my application, wall thickness, unlike in tableware or artistic work, is entirely unrestricted. If the walls must be 1.5 cm thick, or 5/8 of an inch...that's fine. It would be wonderful to hear the whole story of Pinnell's tests - and know, perhaps, just which earthenware clay was used and its firing. I'm searching for it.

 

Earthenware's porosity is key to function in our case - and not due to water absorption or such, but rather, because the porosity contributes somewhat to a more wooden-like (hardwood though) tone. High homogeneity to the body - often desirable among potters - works less well for us, acoustically, it is seems. However, homogeneity would seem to impact strength, which is important.

 

One clay which seem to present model ideal properties is the famous Yixing "purple clay" of china, prized for its special attributes in brewing tea, but significant for our purposes because, as scholars on the subject have noted, Yixing purple clay seems to possess the strength of stronger clays, typically stoneware type clays, along with the porosity of earthenware, valued for absorbing and then transferring the tea leaves flavour.

 

I must learn more about Pinnell's tests.

 

Hope to find more answer! Thank you.

 

 

 

Luca, your first assumption is incorrect. Grog weakens the clay body. Grog gives clay strength during throwing and has other benefits but actually weakens the fired clay but only a little and depending on how much grog is used. A year or so ago a well-known potter's, Pete Pinnell, college class tested a lot of clays for strength and his test and others proved that clay strength depends on the clay reaching full maturity, not how high it was fired and Pete's tests even showed that earthenware was stronger than any of the other clays (cone 6, cone 10, etc.) tested. There were some who disputed the earthenware part of the test's accuracy later because of a thickness issue. But, my point here is that grog and other aggregates weaken clay so if you want to make your clay stronger get rid of the grog and other aggregates and fire to full maturity of the clay. Do not depend on what the clay maker claims. You have to test. While you're testing to see where the maturity point is, you could also test other earthenware clays to find a stronger one.

 

Jim

 

 

 

 

[Guest JBaymore]

It's really very surprising that Pinnell's tests showed earthenware to come out so strong, almost flying in the face of all assumptions about stoneware strength vs earthenware's, even if this was a function of wall thickness.

 

 

I teach technical ceramics at a college and this result has been confirmed by my students tests also. It has nothing to do with wall thickness..... what is used for this is a MOR test (Modulus of Rupture). The calculations for this test take the wall thickness completely out of the equation.

 

When you pursue this kind of testing in a systematic way you will find that stonewares, porcelains, and earthenwares as a general type of "classification" can each have MORs all over the place, depending on bopy formulation, forming method, and firing schedule. Body development is a highly involved bit of work.

 

 

 

BTW... Yixing clay is NOT fired in the earthenware range. It is a high fire clay.

 

best,

 

.............john

[luca 0]

So earthenware, of the right body and firing procedure, exceeds stoneware's strength in a certain manner – which for my purposes would indeed be fortuitous. I can't help but wonder why the general perception of stonewares among laypeople as offering greater strength persists? Presumably this is related to specific types and measures of strength, i.e. torsional or compressive, in addition to being dependent upon the particular bodies of each clay and its treatment. These particulars will be of great interest to me, and I hope to track these down online. The end goal is understanding in order to arrive at a body and a process which meets my needs for a largely low resonant, non-vitreous still porous body, and strength which surpasses that of, say, terracotta used in pots. Certain forming methods are in, i.e slip casting, which has been used in successful prototypes to date working well, while hand forming is too imprecise. I lack familiarity with other methods but know that control over wall thickness and repeatable results will be quite important. I believe there's a form of press moulding that yields precise wall thickness?

 

I hate to appear to the forum with so many questions at once, however, I have been engaged in informal research and prototyping for well over a year, and have done my utmost to familiarize myself with the world ceramacists (and to an extent ceramic engineers) live in. A recent bit of research involved one of the only published texts on a related field to ceramics and acoustics: ceramic instruments in Barry Hall's "From Mud to Music": http://www.amazon.co...l/dp/1574981390. The author generously agreed to share additional knowledge in a recent phone chat. Close listening to recordings of musicians who perform with these instruments made of various earthen and stonewares, and porcelains, was also informative.

 

What the information in this text and the recordings suggest, are a number of properties which ceramic instruments typically posses and which are enhanced by the their makers' techniques and approaches, can serve as a basis for what to do in my application, though often in precisely the opposite way. For example, ceramic instruments, like non-ceramic ones, need to resonate/vibrate, thus more vitreous ceramics are often chosen. I seek to go in the other direction, reducing resonance.

 

I must come to understand what the specific formulations of eathenware and their process of firing were, which succeeded so well in the tests results described above, and why.

 

Kind regards,

Luca

 

Side note: Yixing purple clay, a high fire stoneware, I've found in my readings, is unlike other clays fired in stoneware ranges because it retains a degree of porosity and does not fully vitrify to the extent of most other stonewares at such temperatures. Its still-somewhat porous, non-vitreous body would seem to lend it some of the less ringy tone of eathenware, while retaining strength, but this is based upon the earlier held belief that stoneware generally exceeds eathenware's fired strength, under question now.

[Guest JBaymore]

A core idea that I was trying to get across was also that you cannot generalize across the board.... particularly in an R+D situation for serious industrial or quasi-industrial product development. I've done some research for industry....... it is a different "beast" from the typical studio pottery or academic studio situation.

 

Multiple factors will affect the "strength" of the body. And there are multiple measures of "strength".... you have to assess WHICH of these are important.... compressive, tensile, shear, ruptutre, brittleness and so on.

 

Bodies are designed for specific forming methods along with otehr characteristics. The same body hydraulic pressed and hand pressed into molds will likely give different strength figures. And so on. Hydraulic dry pressing is one option for what you are possibly seeking, with VERY high possibility mathematical size and strength tolerances.

 

I'd suggest that you contract the services of a ceramic engineer with skills in clay body development. Even the most skilled ot us "tech weenies" in the "art" field do not have the background that someone with a M.S. in cerramic engineering would have.

 

best,

 

 

............john

 

 

 

PS: yes Yixing clay is not fully virtified. Many clays usee in Asia for stuff like teawares are not.

[Frederik-W 24]

You are essentially after accoustic results with the characteristics that you mention, i.e. it should not ring, low resonant etc.

It makes much sense to me what you find, i.e. a non-vitreous still porous, low fire body is better.

It is all about how sounds travel through different bodies.

Accoustically speaking, the "softer", more "spongy" a body is, the more it will absorb the sound and the less likely it is to vibrate.

That is why hitting a glass, porcelain or metal bowl (sharp ring) is different to a wooden or terracotta one (dull thud).

I think organic grog that will burn up (like ground coffee), leaving small pockets, would help in absorbing & dispersing the sound. (Sounds travel better through solids than gas). However for slip casting you will need very fine grog.

 

I therefore think it will be difficult to get both a hard body and an accoustically dampening one, because they are mutually exclusive. That is what the science says.

(I am not disagreeing with the expert results that say that you can get very strong earthenware, I just think that the harder (more solid/rigid) it is, the worse it will be accoustically, because sound travels better through harder/denser bodies.

 

Wikipedia:

"Sound travels faster in liquids and non-porous solids than it does in air. Sound waves in solids are composed of compression waves (just as in gases and liquids), but there is also a different type of sound wave called a shear wave, which occurs only in solids. ...The speed of a compression sound wave in solids is determined by the medium's compressibility, shear modulus and density. The speed of shear waves is determined only by the solid material's shear modulus and density".

"Thus the speed of sound increases with the stiffness (the resistance of an elastic body to deformation by an applied force) of the material, and decreases with the density"

 

Note that "sheer modulus" is the rigidity. It would be wise to speak with a physicist or an accoustic engineer on how sound travels in different bodies if you have not already done so.

 

"Strength" depends on function and use. Is the instrument going to be dropped often ? Hardness often goes with being brittle.

Maybe the musician should respect the ceramic nature of the instrument, after all if you drop a violin you would also expect it to be damaged.

I would be inclined to make the body of your musical instrument thicker in order to get more "strength", rather than making it harder and loosing accoustic properties.

[luca 0]

John, Frederik, and Jim,

 

Thank you so much for your input, which, despite the length of time that has elapsed to this response, have been quite something for consideration. I'm currently based in Taipei, and Taiwan has very strong traditional ceramics culture, as well as being savvy in modern technology with a highly educated populace. The first ceramic engineer (MSc) I've come into contact with here was someone who had worked in the same field of acoustics. To an extent, his exploration in our specific common area began and quickly ended with porcelain, and it's evident we share fundamentally opposing views on the value of different ceramic materials in our application. In his view, any particular ceramic material has a minimal impact on sound, to such an extent that it is entirely insignificant, upon the sonic character of the device. Our opinions diverge not because I do not see the impact on sound as relatively minimal – it can be described as such – but I would not, cannot, go so far as to characterize it as utterly insignificant and undetectable. For him, the impact of the porous nature of earthenware or any other attribute of low fired clay upon sonic performance is nothing more than a distinction without a difference. He's content with porcelain.

 

I counter this with two points: there's something in my subjective experience of listening to identical enclosures made of a porcelain and earthenware in the same environment with the same external equipment, that cannot support his claims, and which he himself does not hear. What humans hear subjectively is not the same as what can be objectively measured, however, it seems the difference is not readily testable with the criteria for measuring sonic performance commonly associated with our device. Testing for frequency response, for example, cannot reveal or convey the qualities of our experience with the materials. Granted, he's fully qualified as ceramics engineer with years of experience, but I come at this from a very different standpoint. My background is in audio, including years of experience mixing and mastering music in a studio environment, and developing audio devices. As a result, we hear differently.

 

As a second point, according the principles associated with emotional design, or kansei engineering, the lack of a quantitative objective difference in performance (which I cannot concede yet) is nevertheless still not grounds for judging the difference in subjective experience as irrelevant. We can be induced into perceiving difference by a variety of means, and the mere perception is in itself, significant for a variety of reasons, from an emotional design standpoint through to marketing, branding, and more. But again, I do not believe the difference is merely illusory.

 

Working down the path developing an optimal ceramic body and forming process in conjunction with a technical ceramics engineer will likely yield the best performing solution, however, there are some additional considerations. Aesthetically, the product would appear to have some characteristics typical of more mass production techniques, and others which place it more in the handcrafted camp. Tolerances must be in the range of +/- 1mm, which by studio production techniques are quite precise, but the surface, the surface texture that is, should be pleasing to the hand and eye, with perhaps some subtle, delicate, mild variations. So the form is quite precise, and must be consistently replicable, but the surface texture, somewhat less so, within limits. 

An example image will go a long way to convey the intended aesthetic, specifically in terms of colour and surface texture.

http://www.aldobakker.com/work/2007/water-carafe/168

http://www.aldobakker.com/work/2005/milk-or-oil-can/114

As this example so captured the intended look and feel of the surface, the designer and Dutch production studio he collaborates with were contacted. The porcelain body they've used was then employed in this prototype, which though perfectly rendering the intended feel (albeit with additional fine texturing applied by sponge on a wheel, adding a subtlety lined whispiness the surface) the porcelain body clearly did not have the sonic properties required:

 

This prototype along with another identical in form but of an bisque fired earthenware were tested fully assembled under identical conditions, with the earthenware sounding much more appropriate. 

 

A fundamental issue also is that the surface texture of earthenware looks and feels as desired only in its unglazed state. A clear gloss glaze deprives it of its texture, yet, for white earthenware, a glaze, or perhaps another clear coating of some kind, seems absolutely necessary in order for the piece to be handled without risk of staining by the oils of the hands. In the works above, both by the artist and the prototype, the tighter material structure of porcelain permits handling as an unglazed piece (in its final state) without any real concerns for stains such as one would have in earthenware. 

 

If continuing down the studio ceramics route as opposed to the technical ceramics path, and conducting earthenware strenght tests along the lines of what Pete Pinnell has done to determine the ideal body type and firing temperature, along with working out consistent forming and the correct surface texture, if through this 90% of the desired result could still be achieved, then that's acceptable. Material strength will be greater in technical ceramics, but I'm do not wish to lose the connection to a warmer, richer texture product (though not too far in the direction of handcrafts) more typical of studio ceramics. 

 

This seems a difficult balancing act, thought perhaps not an impossible one. Apologies for the length, and all thoughts and comments are greatly appreciated. 

 

Kind regards,

Luca

[MMB 17]

Lot of reading here and I havent even finished my coffee. Didnt someone state in a thread recently that a 50/50 mix of Red Art and XX Saggar Kaolin creates a pretty strong fired body. Not sure what fired temp it must reach. I remember they said it could be dropped, mind you this is just a statement made not fact, from 5 feet and hit the ground without chipping. My 2 cents. Ive read enough today but will be back later because you inquiry intrigues me.

[Guest JBaymore]

The tight tolerance forming method is likley simple once you seettl on the design...... jiggering or jollying would give you the surface qualities of plastic clays.  Subtle surface variations and assymetry can be built into ONE surface of this process. 

 

best,

 

.............john

[Guest JBaymore]

Spent a lot of my very early life in recording studios as an artist (BC.... before clay....... playing professionally starting at 13.  Yeah... the 60's   Far out man.).

 

Since you are in product development and can't openly share a LOT of infomation here... I realize it is difficult to effectively communicate about this project.

 

One thought comes to mind with me though....... while YOU are aware of the sonic differences, will the MARKET respond the same way?  Have you done any focus group work to see if the distinctions that YOU find important are AS important to your target market?  If not.... amybe you are chasing a detail that is blocking develiopment that would NOT block marketing and sales.

 

best,

 

..................john

 

(Spoken by one whose hearing is SHOT from being a stage performer.  The guy who did the Who's souind system did ours too.)

[luca 0]

Thank you for the replies.

 

John, yes, it's certainly correct to suggest that the distinctions in sonic quality or character would be lost on the general public, however, the design is intended for high-end users and utilizes premium components for a unique product in small scale production, and to carry a price that reflects all of this. 

 

The design concept received support for research and development under a UK business incubation program which did involve reviews and focus groups. Continuing with porcelain, as a few others have done in this narrow field, solves the issue of surface stains from handling on the white unglazed body, but the primary reason other groups may gone with porcelain (commonly glazed) was likely due to the assumptions regarding its strength, something which Pete Pinnell's testing has debunked.

 

The low-fired clay bodies offer the key benefits of a level of workability post-firing, and most importantly, a sound that is largely devoid of the more unpleasant glassy resonances found in vitreous materials. Indeed, from a personal standpoint, I cannot get behind such a compromise as those unpleasant resonances in porcelain, particularly for product that is to deliver exceptional performance. Aesthetically, though unglazed white porcelain was attractive and possible to use without picking up hand oils from handling, with earthenware this is not the case. A gloss glaze applied to earthenware largely obscures the material's identity and its unique properties, almost leading one to assume it could be another material (metal, plastics, etc.) that commonly appears in similar products with a perfectly smooth glossy surface. I wish to retain the honesty of the material, such as one would with many woods, where it would be almost criminal to obscure its surface in paint or an opaque glaze.

 

In terms of tolerances, there are cutouts and holes in the piece that need to match up precisely with other components. What advantages do jiggering and jollying provide in this area over continuing with slipcasting? I see how wall thickness can be handled with a more consistent precision in jiggering and jollying, however, it's really in the firing that the variations appear, ones that can require the cutouts and holes to manipulated further post-firing. Moving into the realm of technical ceramics and other forming methods (compression?) may avert this, but then we may lose the warmer aesthetic we desire.

 

To poster MMB, thanks, and are you aware of professor Peter Pinnell's extensive work on the subject of earthenware strength? Is this approach of adding redart and kaolin, claimed, to your knowledge to be reportedly more effective over what Pinnell has found? Perhaps they may work in conjunction. 

 

Kind regards, 

Luca

[MMB 17]

Your Welcome. No I am not aware of Peter Pinells work, but I shall be looking it up out of curiosity. My 2 cents came from, after looking for certain posts, the Red Art thread.

 

http://ceramicartsdaily.org/community/topic/4503-redart/?hl=saggar

 

Perhaps reading the responses in there from those who have used the mixture you might have some other people that will have something to offer to your research. I believe most of it was stated within the first few posts, but it is brought back up throughout the thread.

[luca 0]

A description of Pete Pinnell's testing process and findings regarding earthenware can be found here: http://groups.yahoo.com/neo/groups/clayart/conversations/topics/331778

 

Two brief points concerning the Redart and XX Sagger combination: This is no longer a very low-fire solution (cone 6), and it was mentioned that the pieces, though supposedly falling an impressive 6 feet without shattering, had "a nice ring" to them....which would suggest a level of vitrification takes place, and would not be favourable for my purposes, acoustically. It's why I pursue the more porous less-ringing low-fire clays.

 

Secondly, the colour...sounds like it's not white? A white, as in the examples above, is desired. A sandstone color with mild variagated marks is a second option, but somewhat less preferable. There's no interest on my behalf in pursuing any other colour options. 

 

I am not opposed to arranging funding for what will surely be a hugely expensive affair if pursuing hydraulic pressing and the more technical ceramics route, but it would be preferable to go to market, at least initially with a slip casted or perhaps jigger/jolly method produced product, in small volumes. Though I find it very interesting, I would almost prefer to delegate much of this testing and comparing of methods, earthenwares, and processes to someone else. I need to streamline this process, run through it more effectively and efficiently. It's a somewhat unsettling state, knowing rather precisely what the vision is, what's desired and much of what's required, although not how to bring it all together and into being. With all certainty, however, the people and resources are out there to achieve it, to lead one to connect all the various aspects. 

[Guest JBaymore]

You likely need the services of a very experienced ceramic body engineer, possibly affiliated with a technical ceramic teaching university.  Hit them with a retainer and a legal NDA so that you can go into full detail on the specifications and nature of the project.  And likely you'll need to invest some serious money in that development research. 

 

You are getting into details with this project that are WAY oustide the scope of the skills of the typical studio artists that frequent this forum. 

 

I'm one of the more formal "tech weenie" types here with what I do at the college level... and while I have done industrial production consulting a bit....... this is not something that I'd want to take on for a number of reasons.... not the least of which is the fact that the set of constraints on the situation seems to be eliminating most all possibilities.  So that tells me that it will be an involved situation that will take much time and testing work to get the results that you are seeking.  A full time job for a good while.  I'm not saying it is impossible at all........ but that given the "hit list" thay you are indicating...... every time you specify one characteristic, you are then specifying another characteristic that makes getting the first one you listed very, very difficult.

 

 

....... however, it's really in the firing that the variations appear, ones that can require the cutouts and holes to manipulated further post-firing. Moving into the realm of technical ceramics and other forming methods (compression?) may avert this, but then we may lose the warmer aesthetic we desire.

 

I don't thnk that this is true at all if the forming methods are done appropriately and the body is correctly developed for the firing range. And if the firing unit (kiln) is working to necessary specs.  Most "firing defects" that potters identify are actually set up by other parts of the process... that only reveal themselves in the firing.

 

I'm guessing that a non-vitreous HIGH FIRE white body formed with dry pressing is likely the place you'll end up to get the blend of all that you want.

 

best,

 

....................john

[MMB 17]

Considering some of the perks with working in the restaurant business, which helps fund anything I do, I do meet a lot of different people. Also to being that I work in Greenville, SC I am surrounded by a lot of industry. I have this family of brothers that come into my restaurant and dine at my bar. They are EXTREMELY intelligent and their conversation alone sometimes blows my mind. One brother, who sadly isnt as regular, works in the scientific field of ceramics. But because his brother (there is 11 of them) Tim comes in the most Ill try to see if I can pick his brain and also see if Tom (the other brother) would be interested in sharing some knowledge. They are always very keen as to what I do because I do clay (which thrills Tom) and I also work a small bit with metal (which thrills Tim). Its like the family made each child choose a separate field to work in. It could be some time before any of this happens but Ill will definitely inquire to him next time I see him.

[luca 0]

John, the insights and advice are tremendously appreciated. I'll try to refrain from posting further here on topics out of the realm of studio potter's ceramics, but you've taken the time to absorb the length of my posts and have a grasp of the scope of the issues, so thank you for the consideration. At this juncture. it's clear that a turn to a full technical ceramics route could be taken, which I'll continue exploring, but as mentioned, I'm also considering that given the "hit list" is somewhat fulfilled by the current earthenware method, it may be acceptable to carry forward to market with a version 1.0 in earthenware for a limited number of pieces. I expect the advantage to be the chance to gauge user satisfaction and feedback on a wider scale, before moving forward to the technical improvements and innovation that necessitate a new phase of R&D. The main risks entail a possibly more fragile piece than technical ceramics and a problematic surface finish, either totally unglazed and prone to stains, or with a marginally acceptable clear matte glaze. If these prove to be significant, then the risk of damage to product reputation may be too great, I'd be forced to pursue technical ceramics exclusively.

 

I'd hope to gather info on the following, and can post these as separate questions to the community.

1. Suggestions on the use of clear matte glazes (which to try, how to use). 

2. How significantly does glaze firing add to the shrinkage of the earthenware piece?

 

I held an incorrect assumption regarding dry pressing, namely that it did not involve firing (and thus avoiding the shrinkage issue, but it ostensibly allows for very precise forming nonetheless). 

 

Kind regards,

Luca

[Guest JBaymore]

Luca,

 

I am not an expert at ALL when it comes to the field of something like sonic vibrations and clay.... but I am thinking out loud here..........

 

At earthenware temperatures there is typically very little "interface layer" developed between the clay body and the glaze layer.  At higher temperatures, this interface usually becomes very mutually involved, and it is impossible tell where the body ends and the glaze begins. 

 

What this means is that basically at earthenware temperatures the glaze is a pretty distinct layer of material stuck (lightly) onto the surface of the undelying layer of clay body. A bit like a layer of paint. 

 

I would suspect that each of these different materials (glass and clay) would tend to react to the vibrations slightly differently. What I am wondering is how the layer of glaze will stand up in the "adhesion" department to long term exposure to such vibrations. 

 

There is a characteristic of low fire wares that over time, sometimes the glaze simply can flake off the body as a defect.  I wonder if this is something that will be exacerbated by being subject to sonic vibrations.

 

Just something to think about in your design and testing regimin.

 

best,

 

......................john

[PeterH 102]

Luca,

I'll propose an answer to this question, hoping that somebody will correct me if I'm wrong.

>2. How significantly does glaze firing add to the shrinkage of the earthenware piece?

 

Basically, if you going to fire to maturity anyway, it doesn't make any real difference.

 

Putting a glaze on a piece doesn't itself add to shrinkage. However if you're firing an already

bisqued piece to maturity (with or without a glaze) it will shrink a bit. I assume that if you then

refire to the same temperature it will again shrink a little, but not nearly as much.

 

 

 

Regards, Peter

[luca 0]

Firstly, John, yes, I do have concerns about the glaze's effects and durability though they are perhaps not quite like you described. Vibration levels in these rather small units will not be extremely intense, but testing has never before been performed with a glazed earthenware piece (though with glazed stoneware, yes). Any possible flaking, though of course still a negative, is just slightly less a concern given that it would be an entirely clear and very matte glaze, and thus the surface does not differ in colour at all, nor greatly in texture either. 

 

My main concern then, is how the glaze may affect sound. It is hoped that it will be minimal/negligible. So for flaking due to vibration and sonic variation caused by the glaze, testing is ahead.

 

Peter, thank you for your input. So I may get a better sense of this, what kind of percentages for shrinkage might typically occur in earthenware for:

1) the first bisque firing (6% or so, for example?) and then...

2) the second firing after glazing (an additional say 3% for a cumulative total of 9%, for example?) and...

3) are the temperatures and time the same for both stages of firing? 

This assumes firing to maturity in the end.

 

It is hoped that tolerances can be reliably kept within +/- 1% after shrinkage, after the entire process has been tested.

 

I should note that bisqueware tends to perform best sonically, but the body must be fired to maturity for the strength in a functional piece. This leads back to the difficult and opposing constraints that John identified, and for which an ideal technical ceramics solution may exist, however, a level of compromise to the sonic performance may be tolerable  â€“ it might be a negligible sonic difference in the end. We'll see.

 

Thanks in advance. 

 

Kind regards,

Luca

[Guest JBaymore]

Luca,

 

A liability concern on the flaking is that the edges of the flaked area will be razor sharp. 

 

And yes, I would imagine that IF the glaze layer "loosens up" on the body over time (it won't be "sudden") the acoustic resonance of the enclosure would likely slowly change also.

 

In the finish firing ("glazefiring") , if it is essentially to the same cone endpoint as the "bisque" firing... the additional shrinkage will be minimal < 1%.  You really will not be changing the body much.

 

You issue is keeping the acoustic resonance of the earthenware open structure NON-vitrified body while gaining the structural integrity you desire.  Even if you remain at "low temperature"...... if you start to vitrify the body (it can be done) ..... the resonance will change.

 

best,

 

..........................john

[PeterH 102]

Luca,

 

I not really qualified to answer your questions, but here are some guess-timates:

>So I may get a better sense of this, what kind of percentages for shrinkage might typically occur in earthenware for:

>1) the first bisque firing (6% or so, for example?) and then...

>2) the second firing after glazing (an additional say 3% for a cumulative total of 9%, for example?) and...

AFAIK you're in the right ballpark, there is also shrinkage from forming to being air-dry. [This is low for dry-pressed ware.]

>3) are the temperatures and time the same for both stages of firing? 

>This assumes firing to maturity in the end.

No and yes-ish. The bisque firing takes the pot very slowly through some risky parts of the firing (e.g. getting rid of chemically

combined water, burning out organics, etc.). The end temperature of the bisque is probably chosen so that the absorbency

of the bisque pot is optimal for dip-glazing. The "glaze" firing can go rapidly through the lower temperatures, but then needs

to reach sufficient temperature -- for sufficient time -- to mature the pot "as desired". AFAIK this ends up with the two firings

taking comparable times. But actual firing schedules depend on the body material, the kiln, and to some extent the whims of

the potter and his/her kiln-controller.

>It is hoped that tolerances can be reliably kept within +/- 1% after shrinkage, after the entire process has been tested.

This is an area where you do need a consultant in technical ceramics. If resonant cavities are involved remember 1% linear

is 3% volume.

 

Regards, Peter

 

You may find acquiring a general reference work, such as The Potter's Dictionary of Materials and Techniques helpful.

http://tinyurl.com/posz3ac

 

Two other surface treatments you might like to consider are Parian and salt-glaze.

 

Parian is a sort of self-glazing porcelain casting body. So named because of its visual similarity to Parian marble.

http://tinyurl.com/oszwb9u

 

Salt-glazing is a specialist technique that gives a very thin surface film of glaze, sometimes pitted in

an orange-peel effect, most usually associated with historic German beer steins and sewer-pipes. Its

a specialist process, which gives a covering that is often attractive and usually very dirt-resistant [and

you do rather want dirt resistance in a sewer pipe].

http://tinyurl.com/o2ac2rb

... perhaps in your case the less obtrusive effects would be appropriate

http://tinyurl.com/pr9hbq5

http://tinyurl.com/plu84dj

http://tinyurl.com/pw5tcb6

[PeterH 102]

FYI http://www.torontostandard.com/culture/horn-of-plenty

 

[Guest JBaymore]

PeterH,

 

My friend MATUMIYA Ryoji-san made a huge one of these that looks like a fish back in July while I was working at Kanayama for the summer.  It works REALLY well.

 

http://blog.livedoor.jp/gallarywando3350/?p=5

 

about 80% down the page.  Sorry all in Japanese.

 

best,

 

...........................john

[bobzchemist 0]

As a cosmetic chemist, I have worked with iron oxides professionally for a long, long time. As a potter, I've only tried incorporating them in my work recently, so I can say that a buff/brown color is definitely achievable (but may take some work to develop). Using an iron oxide wash made with the micronized, sub-micron, or even nanoparticle size oxides used in high-end cosmetics (much, much smaller than commercial iron oxide washes) should allow the iron oxide particles to seal up some or all of the porosity of the outside surface of the bisqueware without affecting the acoustic properties or strength of the clay instrument. A pleasing variation in exterior color could be achieved by either brushing or spraying a somewhat variable combination of oxides onto each piece. Restricting the porosity should reduce the oil-absorbency you are concerned with.

 

Depending on how high you wind up firing these objects, burnishing the outside surfaces might also work (although I don't believe burnishing would lend itself easily to mass production)

 

Let me know if you want sources of supply for these specialized oxides.

 

Bob Zonis