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Specific Gravity - Cones - Variables?

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I have been thinking that one thing I don't read much about is specific gravity in glazes.  There are so many comments that this glaze of that does not travel well.  People don't get the same results as pictured.  Cone temperature is a variable that everyone thinks of, set a cone pad in various parts of the kiln to make sure the proper temperature is achieved. 

 

Yet one variable that seems to get little mention is the specific gravity of a glaze.  Talk about testing the glaze by dipping a finger in the glaze to see if it's the right consistency is very imprecise.  Talk about putting a thin coat or heavy coat is equally imprecise.  Yes, people who are experience can make these judgments on glazes they know and use.  But, for individuals like me relatively inexperienced in glazing and glaze controlling for as many variables as possible can offer me some hope of replication and success in glazing. 

 

So -

1) why is it specific gravity receives so little discussion? 

2) could people start to give specific gravity when talking and writing about glazes?   

 

 

Specific gravity is very easy to do with a digital scale and a graduated cylinder that is marked to 100 ml.  If 100 ml of wet glaze weighs 142 grams then the specific gravity is 1.42   

 

1 mil of water weighs 1 gram.....

 

I don't want to get so precise that its not practical, but, when doing a glaze test or starting to use a new glaze perhaps trying to control one more variable may not be a bad thing.

 

Thoughts?

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An even easier way to control SG is with a hydrometer.

 

If I recall correctly, John Britt's books on glaze formulation give recommended SG for each type of glaze he discusses.

 

Controlling SG is something any serious potter would want to do, in my opinion.  It is true that with experience, you get a pretty good idea of proper glaze consistency.  Also, I tend to use glazes that turn out well even if application is a little thin or thick.  I have no patience with fussy glazes, which may be a character flaw.

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I agree a hydrometer is easy - but I have read that it can be inaccurate.  I really have no way of knowing if this is the case or not.  Putting a 100 ml in a graduated beaker and weighing is rather simple to me. 

 

Agreed, about fussy glazes. 

 

My question was about the reasons glazes present problems for us to duplicate.  My thought is that if we can cut down one variable it might make it easier to duplicate glazes of other. 

 

If a potter is less than accurate about measurement and if there is disparity in specific gravity can this throw off a glaze we make from a recipe?  IF we can work with the same specific gravity would this not help? 

 

Once we get a glaze that works for us and we use it enough to learn the properties then I can see getting a bit less fussy in working with the glaze. 

 

So - could those that post recipes in the forum for others to try give us specific gravity?  Or, if you try a glaze, report the specific gravity used if it works or not.  More information seems to me to be better than less information.

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I think you may be missing the point a little as the SP is really the end game to getting a new glaze ready for production.

 

Once your test of the recipe has established (in your studio, with your clay, using your method of brushing, dipping or spraying) what the SG should be then you can note the specific gravity for the next time you make that specific glaze or need to recondition it. It is going to vary so widely that anything noted with the recipe is simply what the author established for themselves and may well be way off what you need.

 

I guess you could use a generic SG starting point and do your test from reading to reading but most potters I think use a visual method to get it almost where they want to be on a new glaze and then test to zero in. All of that said though I think you will be surprised just how consistent you can be just from visual methods. 

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Testing, ca't get away from it Differetn kiln, different clay, different Bisque temp, differnet glaze app., different water, different surface, different pos. in kiln, different firing schedule, up and down........... SG for when you've tweeked and have what you want if you cannot rely on your visual/intuitive feel., also the thingie of the speed of flow..... read it here some where

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Yes, as Babs points out, viscosity can vary independently of SG, depending on glaze ingredients, which may have flocculating or deflocculating effects. Viscosity can affect glaze thickness if dipped, as a glaze that runs off the pot more slowly has more time to build up thickness.  Some glazes will change viscosity over time, so that the proper SG when first mixed will not be the proper SG after 6 months.

 

I haven't heard that a hydrometer is inaccurate enough to be a problem, but maybe I'm wrong.  It's usually considered accurate enough for wine making.  You might consider also that a physical measurement may lead to inaccurate results due to the heavier ingredients of the glaze settling , and therefore not being scooped up to be measured.  Some glazes start to settle almost instantly (but that's another case of fussy glazes up with which I will not put.)

 

The whole question of glaze thickness is fraught with subtleties, I think.  Different glazes do take different thickness to provide the optimal effect, but that optimum can vary widely among potters and their intentions.  Some runny glazes must be thinner so that application can be done in layers, so that there is less glaze toward the foot of the pot.  If your technique involves layering different glazes, or different application techniques, or even different colorant oxides, this will affect optimal thickness.

 

Like everything else involved in ceramic technology, it gets more complicated the deeper you look.

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Stephen wrote:

"I think you may be missing the point a little as the SP is really the end game to getting a new glaze ready for production."

 

Perhaps my post was a bit "imprecise" and I apologize for that.  I understand that for one potter the specific gravity is an end game variable for one person.  What my question revolves around is when someone presents a glaze recipe for others to use.  That potter is presenting a glaze that is at their end point.  I am sure I am not alone that when If I see a picture of a cone 6 glaze fired in oxidation accompanied by  a recipe I want to give it a try.  Then I go to the studio and mix up a 100 gr batch I am at a starting point for me with this glaze.  Yes, type of clay, temp of bisque firing, water, chemicals from various sources, glaze firing schedule, kiln, and a host of other variables do enter into the equation.  The thrust of my question was, would providing specific gravity "help" eliminate one such variable?  "IF" I have your end point as my starting point would that not make sense?  

 

Will I ever be able to control for variables in other studios? No, of course not.  But, this is one variable that could be eliminated by publication of specific gravity along with any recipe would be of help, at least to me.  The more information the better, bisque temp, firing schedule, etc.

 

"Once your test of the recipe has established (in your studio, with your clay, using your method of brushing, dipping or spraying) what the SG should be then you can note the specific gravity for the next time you make that specific glaze or need to recondition it. It is going to vary so widely that anything noted with the recipe is simply what the author established for themselves and may well be way off what you need."

 

Exactly!  But if I am looking at a picture in a book or online then I would like to have a glaze that looks like what the author has offered.  So their end point becomes my starting point.  As I test, of course I can test and adjust to the particular needs of my own studio and my own unique variables. 

 

"I guess you could use a generic SG starting point and do your test from reading to reading but most potters I think use a visual method to get it almost where they want to be on a new glaze and then test to zero in. All of that said though I think you will be surprised just how consistent you can be just from visual methods."

 

This is precisely where my question comes into play.  As I have stated above,  your end point is my start point.  I don't have the experience to look at a mixed glaze and determine if I need more or less water.  Specific gravity, which is a quick measurement, allows me to hone in on one variable that can be controlled in a glaze test.  This is a more precise way of getting a glaze ready to us - what you and I see as thin cream or how a glaze covers a knuckle can be different. 

 

Many of us buy books, and scour the internet to improve our skill level and to find glazes that appeal to us and will work for us.  I look at the information in some of the pottery books I purchased back in the 70's and compare them to the new book of John Britt on mid fire glazes and the amount of information is leaps and bounds ahead.  For me this is an example of how current practitioners are increasing the body of knowledge in a subject. 

 

For me - and perhaps others - having a starting point for specific gravity is an important tool to reduce one variable.  I know I can never eliminate all variables but why not control for what we can?

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Exactly!  But if I am looking at a picture in a book or online then I would like to have a glaze that looks like what the author has offered.  So their end point becomes my starting point.  As I test, of course I can test and adjust to the particular needs of my own studio and my own unique variables. 

 

 

I think once you go through the process a few times it will make much more sense to you . Usually when you are new to a discipline a glaring fault is often more complicated than it first appears. Folks have been passing around recipes for eons and I think SP is rarely included.

 

Some of the old pros may chime in but IMHO I think it's mostly because its of little value for all the reasons already mentioned (Ray really did a great job detailing in his post :)).

 

If you start with a set of someone's end numbers then it is much more tedious to back it up to what you need. If you want to do anything other than visually thin until its close then I would advise you to pick a generic SP reading on the low side of most glazes for your application, say 1.3 for dipping, then test from there but ya still have to decide where that is taking you before yo test or you are going to be doing a gazillion test to hone in on a SP.

 

Once its to a heavy cream consistency (I'm sure you can tell this) then it take a lot less water than you think to thin it down to brushing, dipping or spraying consistency. If you err a little on thick side then when you test you can make an adjustment and then retest. At that point all things being equal going forward then the SP is golden and I would note it for the next time you make that glaze or recondition it.  

 

One thing I would point out is that this is not a chem process where each factor has to be just perfect or the whole thing fails. Being careful is good but there are a lot of good numbers that will produce beautiful pots and they are based on what you as the artist want from the glaze and you need to establish this.

 

On a side note; I bet those authors in the 70's knew their glazes just fine and I would not hesitate to try their recipes because they didn't note the SP. 

 

Anyway have fun with this and if it seems fun then grab some glaze software and delve into it a little further.

 

 

...if this is driving you nuts then wait until you see that the colors in those beautiful books you are seeing may/will change, often dramatically, once your firing them in your studio. Glazes are not paints and unless you are using stains the color is often based on everything coming together in the kiln to produce a certain color and when it's different it more often than not is a color that is not remotely similar to what it is billed as being.

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For those of you who wish to keep records of your glazes' specific gravity, but do not have a fancy graduated cylinder, Have No Fear.

 

You can use anything that can hold liquid which will fit on your scale. Some things to think about.

 

1. The container needs to be such that you can fill it to a consistant level. If you choose something that you just fill all the way to the top, the container needs to be easily cleaned on the outside, so the measurement is more accurate.

 

2. The container should have a place that can be easily used to record the water weight of the proscribed volume

 

3. I think it a good idea to stick with containers that won't break easily, but that's just clumsy old me.

 

Here is what you do: Fill the measuring container with clean, room temp (don't have to be super accurate in the temp. here) water. I just use the rainwater I collect for glazes. Next you mark the level of the water so that all your fills will be to this line. Weigh the water and record that measurement, on the container if possible, with a waterproof marker.

 

That weight will be your "one". Specific gravity scale is based on water having a value of 1, so heavier suspensions will be greater that 1 and lighter liquids will be less than 1.  The formula for s.g. is weight of unknown volume / weight of the same volume of H20. Pretty simple.

 

So with your dixie cup, or pickle jar, or pint scoop, you would divide the weight of your sample, by the weight you recorded in waterproof marker on your container!

 

Remember, if you are mixing your glazes with water, the specific gravity of any of your glazes (as far as I know) will always be a value larger than 1.

 

One more thing. If you keep that measuring container forever, you can simplify things for yourself and just record the actual weight of your glaze rather than calculating the s.g. Slip casters call this the "pint weight". Recording this gross number on your glaze buckets allows you to do less figuring and more potting, but don't forget that initial s.g. reading or you'll be sorry when the cat knocks the "specific gravity mug" off the top shelf in your studio.

 

Has anyone mentioned yet that unless two people bisque to the identical porosity, the specific gravity for one will still only be a ball park number for the second?

 

Hope this helps, and sorry if someone has mentioned this earlier in the thread. Sometimes I don't read carefully.

 

Peace out,

 

Taylor in Rockport TX

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Stephen wrote: "Usually when you are new to a discipline a glaring fault is often more complicated than it first appears."

 

I am not really that new to mixing my own glazes, I have been mixing glazes for around 10 years or so.  I do not consider myself a glaze expert, but I do know what works for me.  Since using specific gravity I have had less problems and better results.  So perhaps others may find it useful as well...... That's why I think it would be helpful to have a specific gravity of a glaze that someone thinks is worthy of publication.    

 

 

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well I did think you were new to mixing glazes since you said in your original post you were inexperienced in glazing, maybe some of my rant will help someone else see an additional perspective  :)   

 

Hey though to be clear I was never ever saying not to use specific gravity. I just take the position that the specific gravity should be established by the potter when they are testing and that a published specific gravity reading should not be used as a starting point.

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Stephen, at 10 years or so of mixing glaze I do consider myself to be inexperienced.  I don't have a MFA or anything close..... well I do have an MS in Criminology but then there is no overlap there :-).  

 

I do know I am getting better, and that's the whole point for me....  I am just trying to figure out ways of getting better and increasing the learning curve for me. 

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Maybe I'm missing something, but if it were up to me making a glaze, I might start with a specific gravity, then see

how thick it was, coat a cup, make notes, add water, coat a cup, make notes, etc until it appeared to coat

evenly, and dry like its suppose to, making notes all along.  The cup that appeared to be the best example

refer to the notes to see what the specific gravity was at the time the cup was glazed and make that the

standard specific gravity for that glaze.  To me its all about the final result, not some number on a hydrometer.

I haven't made glazes in a long time, but when I did, the recipe was followed, mixed, dip the hand in, look for thickness,

sieved a couple of times, dip the hand in checking for thickness and put back on line.  We were more concerned

with the final results than specific weights.  We didn't know about hydrometers in the art dept... We used them

at the automotive shop checking batteries and radiator fluids.

Alabama

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I have seen plenty of pottery that is absolutely criminal, so maybe an MS in Criminology is more useful than you think in this business.

 

I regularly use Specific Gravity as a RELATIVE measure when testing iterations of the same glaze that I am trying to tweak or improve.  When each new batch is very similar (but not quite the same) to the previous one, you would hate to have uncontrolled/unobserved variations in glaze thickness or density trick you into thinking that something else had been the source of the change.   So like any testing you try to lock down as many variables as possible to see what is really changing.   

 

I guess when discussing viscosity we have to be clear whether we are talking about the melting characteristics of the glaze, or of the rheology (fluid characteristics) of the glaze slop.  I do a zahn cup test when mixing every new big batch to make sure the slop is consistent. 

 

Finally, I would be careful using water as a way to thin out glazes.  Yes plenty of times this is the solution, particularly after a lot of evaporation or absorption losses of water.  However, in the first instance much smaller amounts of a deflocculant is a better way to adjust the glaze slop fluid characteristics.   Darvan being a high end solution, and soda ash and sodium silicate at the workaday level.   

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Hi curt, in response to your advice to be careful about using water to thin out glazes. This has always been a of confusion to me. I am referring to my initial mixing of dry materials in this post and I always start with about 80/85% of the water I think I will need and then I let that combo steep overnight and then slowly mix in more water as I sieve with visually inspection for thickness and test fire. If needed after my test, I thin with a little water and re-test and then put that glaze in production when it is behaving itself on the test tiles. 

 

I am currently working through my entire lineup of 35ish glazes, noting both a drip tool count reading and a specific gravity for each one so I can be more consistent when I make another batch and/or recondition. I'm down to the last 5 that I am working on thickness as they are still a little thick and need to be thinned. 

 

Now I have been told to be careful about using deflocculant unless the glaze is one that seems to stay thick even as water is added. I don't currently have any like this so I just use water. Now we keep litds on them in between using so they do not loose very much moisture. 

 

If I understood you correctly you are saying that when reconditioning glazes that have lose moisture through evaporation you recommend using a deflocculant instead of adding water to return to the right consistency?    

 

Am I understanding you correctly?

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SG can help with consistency and repeatability in your studio, but it will do nothing to help when a glaze recipe travels to another studio. There are too many other variables that come into play. Even if you knew the SG and how long it was dipped, different clays and different kilns will affect the results. Plus everyone has their own preferred glaze viscosity and application process, so the SG that you use may not work with some else's process. I get what you're getting at, but glazes have to be tested, tested, tested, even if someone else has used it a million times in another studio.

 

I never use SG or a hydrometer in my studio. With 15 class glazes that sit open all day during glazing week, I just don't have the time to repeatedly test and adjust all those glazes. Even when it comes to my own work, I don't do it. But I know my glazes well enough to make adjustments to the viscosity of the glaze by eye, and adjust the application as needed. Getting to know a glaze well takes a few firings, so I always tell my students to keep notes and try a glaze more than once.

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Hi Stephen,

 

No you had it right in the first place, sorry to be confusing. If the glaze has lost water over time then by all means add back water.

 

I was really referring to the initial mixing process of the batch. A common example where this can be an issue are glazes with lots of clay in them, which even when their specific gravity seems right (eg, 1.4 or 1.5) can sometimes feel too thick and gluggy and not apply well. Lots of glazes have lots of clay in them, and many recipes suggest adding even more clay in the form of, say bentonIte, to get the glaze to suspend well in the bucket between uses.

 

This is where a zahn cup test will be valuable in showing whether the fluid flow is right or not. Do a zahn cup test on some other glazes that are known to apply well and compare. If your new glaze is too gluggy even at the correct specific gravity then a defloculant may be the right thing to add rather than water. Too much water can over dilute your glaze, and you may find that that your bisque pots are having to soak up too much water to get the right ending dried glaze thickness. This is particularly an issue when spraying transparent glazes where the last thing you want is a run or a dribble from over spraying. Too much water can also cause a glaze layer you have already applied (say, a liner glaze on the inside of a mug) to pull away from the bisqued surface and flake off. Not good.

 

Neil, I take your point that in a mass production (er, class?) kind of situation this kind of measurement is impractical and probably doesn't yield much benefit for well known glazes as too many other variables are in play. However, I am sympathetic to the idea of knowing whether SG is, say, Low, Medium, or High, with a new glaze recipe. As you suggest, the starting point with a brand new glaze is always a relative process with lots of testing required

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