hanee Posted January 5, 2015 Report Share Posted January 5, 2015 I'm interested in learning more about how grog affects working strength. In most articles I can read online about grog the primary effect seems to be to aid in even-drying. Most articles talking about strength and grog also are seem to be primarily discussing it's effect on the fired product. For example, the article on digital fire about grog doesn't even mention it's effects on working strength. On the other hand, I've always experiences that a grogged clay will be stronger in-working than an ungrogged clay of the same body. I've heard this reaffirmed in manufacturers descriptions and from othose I've known in the past, also. But I'm not finding any thorough source explaining how grog amount and coarseness relates to working strength (in terms of supporting an outstretched arm, for example). I'd like to determine an ideal percentage and coarse-fine ratio to use when optimizing an earthenware body for working-strength versus any other feature (fired strength, dry-ability, etc). Anyone care to explain this or refer me to a better source than the one's I'm finding? Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted January 5, 2015 Report Share Posted January 5, 2015 Good question. I would think it is something to do with the larger sand particles interacting with the platelets of clay in some way making it stronger and increasing the friction between all particles. I started doing a search for 'particle size in clay and working strength' and found this article http://www.cdc.gov/niosh/mining/userfiles/works/pdfs/sseoc.pdf that seems to talk along the lines of shear strength of clay/soils. I would think the ability to hold up an arm would be down to shear strength. I had a quick read and was lost in a few of the terms but if you have the time it could go in the right direction. There was one interesting part (page 7) with a nice diagram. It seems they say under 50% grog the strength is all down to clay and over 50% there is friction between the sand particles themselves. If you could find the article they talking about it could be a good. I am sure the workability depends on a lot of different factors, including what your needs are. Try searching for 'vallejo and zhou 1994 clay kaolin sand' Link to comment Share on other sites More sharing options...
Guest JBaymore Posted January 5, 2015 Report Share Posted January 5, 2015 There are many types of "strength". And many states of clay. No one answer here. MOR tests and vibrational tests and impact tests and crushing tests would tell you stuff. Depends on what you need what direction in body formulation you should go. best, ......................john Link to comment Share on other sites More sharing options...
hanee Posted January 8, 2015 Author Report Share Posted January 8, 2015 Highbridge, that's really fascinating. If we're both understanding this particular diagram correctly that would mean that the reputable sources I'm reading are only speaking of grogs impact on shrinkage/drying issues because it perhaps has no significant impact on green strength in the concentrations we usually use it. Do you know when they say % are they usually referring to by-volume-% or by-weight-%? I recently got a copy of Daniel Rhodes' Clay and Glazes for the Potter, based on recommendations on this forum and the only references to grog are in terms of shrinkage/drying. Seems counter to everything I learned in my figure modeling classes way back when, but I expect potters know more than sculptors on this one. JBaymore, the kind of strength I'm looking to optimize for primarily is that of an outstretched limb. Secondary to that is compressive strength (say, all the weight of a figure on one leg). Both in the early-green state (i.e. working consistency). Empircal testing is great, but it's nice to have some knowledge to give a bit focus to otherwise random permutations, if such knowledge can be had. Link to comment Share on other sites More sharing options...
neilestrick Posted January 8, 2015 Report Share Posted January 8, 2015 In my experience, grog has a negative effect on the strength of bone dry and fired clay. Take a piece of smooth fine-grained grogless stoneware and a grogged one of equal thickness and see which is easier to snap in half. The main benefit of grog, in my opinion, is that it increases the plasticity/workability of moist clay, and helps reduce shrinkage, cracking and warpage during drying. One of the ways it aids in workability is that it increases particle size distribution. The greater the number of different particles sizes in a clay body, the more workable (generally) it tends to be. As for shrinkage, cracking and warping, it is my understanding that grog, which doesn't shrink, causes micro-cracks as the clay shrinks around it, therefore reducing larger stresses that cause problems. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted January 8, 2015 Report Share Posted January 8, 2015 It is by % volume. Remember that the articles I can seem to find are about dam building and shear force in relation to that so the data can only be interpreted for clay for sculptural use. I did read another article that seemed to say anything over 20% had a noticeable increase in strength. Lots of the test are making a three layered stack of different clay/sand ratios and compressing them, then doing something strange with water and saturation. I still can't get my head round all that they are saying. They all seem to be using different variations of water in their experiments. Some are using quite large rocks (over 15cm) Here is a graph I also found that shows many different tests. As you can see the lines are all over the place. I think the best line to look at is the Miller and Sowers as they use the smallest particles with a max of 5mm. The strength increases slowly up to 20% then drops between 20 and 40% and increases again the more you add. I am a little confused on which line is which and what all the headings mean. I have read that grog changes how the stresses act along the clay. There is other info on void space and how that can change things. The more plastic the clay also seems to change how the particles deal with stress and the varying size of particles can also make a difference. Lots of articles that look good have been behind pay walls so I haven't been able to read as much as I would like to. Nobody seems to have done much research into it from a sculpture/potter point of view so it is hard to know if all this info translates across. It all seems rather complicated how a clay/sand matrix interacts and with most things ceramics, it depends on a lot of different variables. If you really want an answer you will have to do some testing of different percentages and size variation. I would guess between 10 and 20 % with a few size variations for a good clay to work with. Link to comment Share on other sites More sharing options...
PeterH Posted January 9, 2015 Report Share Posted January 9, 2015 Forgive the digression, but have you considered paperclay? I am under the impression that several sculptors had found paperclay a useful and relatively forgiving medium. http://www.ian-gregory.co.uk/paperclay.html http://judynelsonmoore.com/wp-content/uploads/PaperClaySculptureHandout.pdf http://www.talleranoitina.es/documents/Paperclay_for_Ceramic_Sculptors_A_Studio_Companion.pdf H&S point, at a demo by Ian it was advised to remove any bubble-wrap or balloons from the figure before firing [if fumes were a concern]. Let figure firm up, cut an access hatch, perform bubble-ectomy, replace hatch. Link to comment Share on other sites More sharing options...
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