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determining target specific gravity for glazes


Beebop
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Hi all, posting two questions in two days, I’m on a roll!

How do you know what specific gravity you should be aiming for when mixing a glaze? some recipes indicate it, some don’t and when i make up my own recipe its up to me to determine. i’m assuming high iron glazes have a higher SG than a clear base, but whats a ball park for these? britts midrange book doesn’t specify SG for clear bases. he also specifies different SGs within the same glaze family without saying why. 

Is equal parts by weight for solids to water a good average? is there a way to calculate the target SG based on ingredients? should i just always aim for a cream consistency from the outset, measure SG, then maintain that SG with water while maintaining consistency with flocc/deflocc over time? 

i suspect i am significantly underwatering my glazes, can someone provide ball parks of what is too high or low and what would be an average SG?

thanks!! 

 

 

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1 hour ago, Beebop said:

glad it’s not just me confused! 

I can't find much information on this at all. From what I've gathered 1.4 to 1.5 is typical for a homemade glaze. I think I'm going to try testing different sp gr around that range.


Someone help us!!

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1 hour ago, offmenu said:

I was about to ask the same question, instead I'll follow this topic :D

 

1 hour ago, Beebop said:

glad it’s not just me confused! 

This has been discussed a bunch and there are actually many components to the answer ……….  I would suggest to start with the link to the article  below first. Sue does a nice job of illustrating the why, the potential variability and then the how.

https://suemcleodceramics.com/how-the-water-in-your-glaze-is-affecting-your-results/

Edited by Bill Kielb
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19 minutes ago, Bill Kielb said:

 

This has been discussed a bunch and there are actually many components to the answer ……….  I would suggest to start with the link to the article  below first. Sue does a nice job of illustrating the why, the potential variability and then the how.

https://suemcleodceramics.com/how-the-water-in-your-glaze-is-affecting-your-results/

thank you !

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2 hours ago, offmenu said:

I can't find much information on this at all. From what I've gathered 1.4 to 1.5 is typical for a homemade glaze. I think I'm going to try testing different sp gr around that range.


Someone help us!!

i’ve been aiming for between 1.4 and 1.5 thinking that’s pretty standard, but everything is going on so thick. i’ve been thinning with Darvan, but I suspect that i might just need to add water rather than deflocculate. i’m concerned though since i aimed lower with my first clear base and it was a thin runny mess! i guess it’s just trial and error and good note taking? 

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2 hours ago, Bill Kielb said:

 

This has been discussed a bunch and there are actually many components to the answer ……….  I would suggest to start with the link to the article  below first. Sue does a nice job of illustrating the why, the potential variability and then the how.

https://suemcleodceramics.com/how-the-water-in-your-glaze-is-affecting-your-results/

thanks for the link. this explains why it’s important, which i have a pretty good grasp on, but it doesn’t explain how you know when the SG is too high or low during mixing, versus when to flocculate or deflocculate. this might be the most mysterious part of glaze making for me, but i’m sure it’s only because i’m missing something very obvious that just goes without saying. i haven’t been able to find solid guidance on this, but i’ll do a deeper dive into previous forum posts and books. thanks! 

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You could always run some tests to take out some of the guesswork. Tall test tiles, approx the same thickness as your pots, bisqued to the same cone. Mix up a couple hundred grams of base glaze with enough water to get it to 1.5 sg. dip a test tile once (count the seconds off) then when the sheen is gone dip the top half of the test tile a second time. Add a small amount of water and measure the sg again and dip another tile. Keep going until the sg goes down to 1.4  If you get 5 different sg's that should be plenty to narrow it down. Fire them as you would your pots and see which is best. Get the sg nailed down first then if the glaze slurry needs it address the question of to floc or defloc.

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1 hour ago, Min said:

You could always run some tests to take out some of the guesswork. Tall test tiles, approx the same thickness as your pots, bisqued to the same cone. Mix up a couple hundred grams of base glaze with enough water to get it to 1.5 sg. dip a test tile once (count the seconds off) then when the sheen is gone dip the top half of the test tile a second time. Add a small amount of water and measure the sg again and dip another tile. Keep going until the sg goes down to 1.4  If you get 5 different sg's that should be plenty to narrow it down. Fire them as you would your pots and see which is best. Get the sg nailed down first then if the glaze slurry needs it address the question of to floc or defloc.

can do thanks for the advice! 

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7 hours ago, Beebop said:

but it doesn’t explain how you know when the SG is too high or low

A lot depends on desired aesthetics, ease of application while using your firing method, your application preference in your kiln. Testing provides results which creates experience which leads to consistency.  As a general rule I like my clear glazes thin but fully covered to get the greatest clarity possible. When I dip I like a three second dip so after testing I settle on 1.38 SPG for glaze A for example. It’s hard to get experience in advance but one can pick up the why parts (as above, I want my clear glazes as thin as practical for best clarity) and apply them through testing to get MY best preferred performance for them. Frustrating I know, but testing with keen observation is often the best way to figure this out for each glaze.

I think there is great truth when Sue said “ Each glaze will have a different “ideal” ratio of solid particles to water. You get to decide how you like each of your glazes best.“ 

 

Edited by Bill Kielb
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We can measure specific gravity fairly accurately, and it is also fairly easy to control. I'm noting SG in my glaze notebook - one less variable! An inexpensive plastic 100 ml graduated cylinder and a decent scale does it. The seven (or eight?) glazes I use most range from 1.36 to 1.48 SG.

Would thixotropy and specific gravity be considered together? Per Mr. Hansen, "The secret to all of this is not intuitive. It involves adding more water and then gelling the slurry using a flocculant (vinegar, epsom salts, calcium chloride) to reach a point at which the slurry is both creamy but also thixotropic. Being able to measure the specific gravity of the glaze slurry accurately is very important in accomplishing this." *

More water, aye that. Does a wetter glaze require a bit longer dip to get the desired thickness? ...a bit more time, a bit less rush? Does one gain back the time (and more) by  dialing in the "gel?".

Measuring thixotropy, perhaps not as straightforward as SG; I stir clockwise (easier on my wrist) to the meter of Positive Vibration, looking for the mass of glaze to turn about three times before coming to a stop, and "bouncing back" altogether. At the other extreme, the glaze will continue turning, where there are significant shears - different currents of speed. Some glazes require more adjustment than others...

The difference is striking. Adjusting/controlling thixotropy and specific gravity - companion topics, in my opinion! Glaze on.  

*from this article: Thixotropy (digitalfire.com)

see also

Thixotropy and How to Gel a Ceramic Glaze (digitalfire.com)

"This will change your life"

Agreed.

Edited by Hulk
range
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ok, i am self taught over many years.   until i saw specific gravity come up on this forum, i never heard of it.  same with LOI.     glaze making was simple and  involved using recipes from other people and testing whether or not they would work on my clay.   water was the only thing i added and i found that about 2/3 water to the weight of chemicals worked well in most cases.   adding water or letting a too watery mix settle and removing the excess water (saving the removed water just in case) was the only variable necessary.    

lately, i see lots of questions from potters about what i consider very advanced subjects.  and what appear to be very technical answers to those questions.   some of the questions are from admitted raw beginners making their first glazes.   why does anyone need to know more than recipe, dry ingredients measured correctly, plus water to try out a glaze?     i know i am out of touch with all this new technology but is specific gravity really necessary just to see if something works???? 

going back to my little corner now.

BTW hulk, if you put your bucket of glaze on your wheel and turn it on slowly, all you have to do is hold a wide stick down to the bottom of the bucket to mix your ingredients.    nice mixture, good lesson in paying attention to what you are doing so you do not paint your entire studio with glaze splash.  music optional.

 

Edited by oldlady
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Specific Gravity is the ratio of density of a substance to the density of a control.

We use the density of our glaze compared to the density of water. Because the density of water is 1, to determine Specific Gravity, we really just need to calculate the density of our glaze.

Specific Gravity = Glaze density / water density = our glaze density

Density = Mass / Volume.

 

Specific Gravity has no effect on the glaze itself, rather, it reflects the amount of water that’s a part of the glaze suspension. Which affects how it’s applied. Due to the way the raw material is applied to the bisque-ware by Capillary_action. 

So a very specific, Specific Gravity is not the most important thing to worry about. It’s just another (although minor) variable you can consider.

 

Flocculants and deflocculants are a whole different topic as they affect the viscosity of the suspension without much affect on the Specific Gravity. 

 

That said, it sounds like you need to add more water. If you’re dipping, you dont wan’t a cream cheese consistency. The amount of water or specific gravity of the other glazes is a good starting target to shoot for. 

Lastly, you can’t ruin a glaze by adding too much water. If you do add too much, all you have to do is let it settle and syphon off some water from the top.  

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Add water to change the SP not Darvon

I like using  the weigh method with the 100 ml syringe that Min spoke abopuit last year. Easier to read than the hydrometer.

Best way to find what works with what glaze is testing-some glazes it really does not matter much. This clears for me are that way -I can tell but how it runs off my hand.(i wear gloves when glazing to save my hands from drying out)

Matt glazes are more critical as are high rutile base glazes and I use the weigh technique to get it right-out of my 15 usual glazes I only measure 3 of them.

Of course experience plays a big role-you learn this over time<My clay is porcealin and glazes are cone 10

Edited by Mark C.
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i have lots of scales graduated cylinders and scales, i understand what SG is and measure it with every batch, so no problems there. i understand the chemical difference between deflocculation and adding water, i understand that deflocculating doesnt change SG and why. but what i don’t understand is how to tell the difference between over flocculation and when something just simply needs a lower SG. this is helping me a lot though, thanks everyone! i will aim closer to 138-140 for clear bases and mix with whatever amount of water is needed to achieve the consistency i need. if that consistency changes while in the shelf, i’ll measure SG to determine if its water loss or overflocculation causing the issue and adjust accordingly. thanks all! 

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4 hours ago, Hulk said:

Measuring thixotropy, perhaps not as straightforward as SG; I stir clockwise (easier on my wrist) to the meter of Positive Vibration, looking for the mass of glaze to turn about three times before coming to a stop, and "bouncing back" altogether. At the other extreme, the glaze will continue turning, where there are significant shears - different currents of speed. Some glazes require more adjustment than others...

this is it! this is what i’m looking for! so helpful thank you!! i will work on this skill :)

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3 hours ago, oldlady said:

 i know i am out of touch with all this new technology but is specific gravity really necessary just to see if something works???? 

It's just weighing the glaze slurry, if you are making glazes you already have the scales so it only takes a minute using a syringe or a few more using a graduated cylinder.

Nope, it isn't totally necessary but for some glazes it makes the results more successful. Add flocs or deflocs and the glaze will look like it's got more or less water in it than it actually does. I know when I started out in ceramics glazes we didn't measure sg, the guys with hairy fingers would stick their hand in the buckets and see how the glaze ran off their fingers, or people went by skim milk, homogenized milk or light cream descriptions. Sure you can do it that way, with many glazes it's close enough that a bit of leeway with water amounts doesn't really matter but for some glazes it does make a difference. A minute or two of work can save a load of pots.

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In the 70s we laerned to use our hands to see how the glaze ran off. Thats how I did it as a collage tec for about 4 years.

Later it was the right hydrometer  which I only used in the touchy glazes

-ruining 35 cubic feet of pots because of runny glaze is no fun (been there done that)

now its  weigh in syringe -super quik and easy

on another note mixing dry glaze to wet it takes a while (a day really to saturate all the small particles) so the measurement for me is taken later after the glaze sits for some time (usually a day)

Edited by Mark C.
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absolutely right, mark!   mix the water and chemicals at least a day before it is be used to allow for that needed saturation of ingredients.  all of these things are learned by experience or advice from someone who freely spreads the word.   i guess that is why i wonder there are so many complicated answers to simple questions. 

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I really like to view it as an accurate set of fingers. In the beginning when you get a glaze that works to your liking, measure the SPG then you can recreate it easily. When I first started, folks describing thick and creamy, light cream, etc….  was helpful but very subjective. I also benefitted from the experience  of spray painting  so viscosity was something I was very familiar with so flocculating and deflocculating for viscositymade some  sense to me from the get go.

All the techniques when learned are intended to improve the glaze application but for me always start with SPG for the reasons Sue illustrated in her beginners explanation and if you want grow your knowledge and experience to include viscosity type modifications do so.

If I was custom painting your Bentley, you likely want to make sure I adjusted the viscosity and checked my fan width etc…  Can I just do it by fan width and familiar pressures and how it pours into the cup, yeah pretty well …………..after 20 years of experience and some interesting failures. SPG in the beginning is a uniform set of fingers not requiring some subjective description of how a glaze flows over the digits.. 100 ml syringe makes this super easy or a 50 ml and double the measured weight. Almost as easy as dipping your fingers and way more repeatable when first starting out.

Edited by Bill Kielb
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this is all helpful thank you all! i think my biggest issue is that i’m working in a home studio by myself and don’t have the experience of others to learn from. i havent been using glazes made by someone else so i don’t know what it’s  supposed to look like and feel like, i cant say oh this is too thick or thin base on past experience, aiming for random specific gravity numbers at random.  books and theory only get you so far and these tactile details are hard to translate through words. but i feel much more informed and empowered to make it my own and take  good notes so i can replicate it. thanks everyone for sharing so much info with us newbies asking the same questions over and over! you’re all amazing!! :wub:

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SG is a measurement that is very much situation dependant. There’s lots of glazes that behave just fine in the bucket, and you don’t have to worry about it. I find it’s much more of a concern when you have glazes that have frits in them, or not enough clay. Neither of those things is usually an issue when you’re working at cone ten, which I think explains the prevalence of folks trained 20+ years ago who haven’t needed to bother with it.

I find with cone six, more glazes are application sensitive, and have a  narrower window where they’ll give you your desired results. The glazes can also sometimes be low in clay, or high in frit in order to be able to use boron effectively as a flux. With more people firing to cone six, discussions around how to fine tune your application to get consistent results are also on the rise. So now we learn about SG and flocculation.

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