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Pugaboo

Hydrometer

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I just ordered a hydrometer and need to get a clear test tube to use with it... what milliliter size do I need to get?

 

Also it came with no instructions... ummm .... how do I use it to rest the specific gravity of a glaze?

 

Yeah I know my blonde is showing. I could bat my eyes and act the dumb blonde (wouldn't be much of a stretch!) and get someone else to do it...but I want to learn this myself.

 

I am beginning to mix more of my own glazes and also buy powdered instead of premixed glazes so would like to get a better understanding of how much water I should be using to do so.

 

Thanks for the help!

 

T

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It will float vertically in the glaze mixture. You can read the numbers on the side just like a thermometer to compare to what your recipe instructs for specific gravity. Pick out a test tube or graduated cylinder after you get it- although I just use my glaze bucket directly. Specific gravity helps you determine how much water you need  in your glaze mixture. For most glazes, I'm usually lazy and mix my glaze with extra water, then sieve, and then wait a day or two for it to settle and then remove enough water from the top & mix it up good again. Instead of the hydrometer to get to the right density, I dip my hand in to see how much it coats my hand. 

You can find videos on you tube about using hydrometers.

I wonder how many people use them to mix glazes every time? Maybe its more important for production potters?

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If you have smaller amounts of glaze buy a plastic graduated cylinder from any web source.A tall one is best about 10-12 inches tall. I use mine in the glaze bucket -just the hydrometer or the cylinder when I have less glaze and it bottoms out.

I also really like to weigh glaze on my digital scale -that requires a 100 millimeter cylinder 

Its bit more precise than the hydrometer .I just tare the cylinder weight off or add it to total then subtract.I wrote the important weights on the wall in studio-I cannot loose them that way. 194 is the glaze I use all the time.

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Just as with the wide-cone-range of clay bodies, there is common misconception about the utility of hydrometers. First, there are 2 types of hydrometer. One measures liquids that are lighter than water, such as alcohol, and the other is for liquids heavier than water. The light hydrometers are used in wine and beer making, and readily obtained. Heavy hydrometers are not as common, you have to know what you need and seek it specifically. Hydrometers are accurate only in non-thixatropic slurries. In thixatropic glazes, I can put the hydrometer anywhere up or down and it will just stay there. Gerstley borate is highly thixatropic, as is neph sye. Consequently, midfire glazes that are likely to have these ingredients can't be measured accurately with a hydrometer. Cone 10 glazes that rarely use these materials might be measured more accurately. Finally, hydrometers do not measure specific gravity in the standard scientific sense. They are calibrated in degrees Baume, which can be converted mathematically to specific gravity, but with the similarity in the range of numbers can be easily confused. When you get a recipe from an unknown source and it is annotated "specific gravity 1.47," is that degrees Baume on their hydrometer or 1.47 grams per cc? The numbers are the same but the density of the slurry is different. My preference is to measure specific gravity with a graduated cylinder or syringe and digital scale.

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I got my hydrometer from a pottery supply house. I did see other versions online as I was searching and wondered at the difference. Now I just need to get a 100ml clear test tube and try it out.

 

I am wondering even with thixatropic glazes if I figure out a number on the hydrometer that gives me the glaze consistency I like if I can then use that number each time even if scientifically speaking it's not a true number? Just wondering on that.

 

One of the reasons I am trying to do this is because I have some glazes that if I do the finger test and get it to read correctly for that they are Too thin snd fire ugly brown. Then again I have other glazes that if I stick my finger in and it looks too thin... that is perfect otherwise I get pinholes.

 

So I guess what I am saying is instead of writing on the side of the glaze bucket...USE THIN, USE THICK, but no way to gauge what is too thick or too thin.

 

So I guess what I am saying is I am hoping that between the hydrometer and the zhan cup I can come up with repeatable recordable standards. Also add into this mix a couple dozen glazes at the art center pottery I am now in charge of but that I am unfamiliar with. I need to get them all sieved (glazes should not be chunky UGH), organized, test tiles made, and easily followed instructions to put on each one so members can know the glaze is at its correct viscosity even when I am not there. Not sure if it's possible but would like to be able to write: use hydrometer (or zhan cup) to test, the number should read ____. Right now some never even test with their fingers, some are confused as to the finger test, others do the test then add too much water, etc. I am attempting to get the glazes to the proper level then maintain them there rather than the huge swings I see happening now that not only waste glaze, they are ruining the kiln shelves and causing pottery failures to happen.

 

Am I asking too much?

 

T

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I use a low-tech homemade hydrometer. Don't think I could live without it. I have one glaze that contains gerstley borate and is quite the flocculator. To measure it I give it a thorough stirring then use the hydrometer before it sets up again. My results are consistent enough for me, and I'm very picky about it.

 

The art center will thank you! Maybe not at first, because some people will resist the idea that things need to be measured. Once you have the recordable standards figured out for every glaze, people will see how much sense it makes. And yes, you will save them lots of money on glaze materials and ruined shelves. It's a big project you are undertaking. And totally worthy. Good luck!

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Once you have the number figured out write that number on side of glaze bucket.

I use both tools but I like Dick said

My preference is to measure specific gravity with a graduated cylinder and digital scale.

I think its easier.

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I have a tall extruded vase (about 12 inches tall) that i put glaze in to use hydrometer. Sometimes a bucket of glaze is not deep enough. I use it mainly for terra sig and naked raku materials.  

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Guest JBaymore

Two measurements are good for keeping glazes consistent.  One is the Specific Gravity (or "Pint Weight" in old potter-speak).

 

The other, and often more important one is the flow qualities (rheology) that are determined by a Zahn's Cup. 

 

You can make both simple hydrometers and Zahn's cups for your personal use.  Not accurate to "standards" but accurate to YOUR particular situation.  If your readings match batch to batch... they are the same.  You just can't compare to published "numbers' or results of others.

 

best,

 

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

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I started using a hydrometer when I was teaching ceramics at the high school level.  With so many students using the glaze it was a good way to teach them the importance of consistancy.  I still use the hydrometer at my home studio (force of habit). I use Amaco Potter's Choice glazes that are shipped in powdered form so I do not measure the raw materials.  I have learned through trial and error my favorite cone 6 glaze works best when the hydrometer is always at the same reading. I just use the hydrometer in the 5 gallon bucket of glaze that I use for dipping. If the glaze is not deep enough in the bucket, I use a narrow plastic pitcher. My glazes often sit for 4-6 weeks in between glazing sessions so I test with the hydrometer each time I start a new glazing session.  Works for me.

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If you have smaller amounts of glaze buy a plastic graduated cylinder from any web source.A tall one is best about 10-12 inches tall. I use mine in the glaze bucket -just the hydrometer or the cylinder when I have less glaze and it bottoms out.

I also really like to weigh glaze on my digital scale -that requires a 100 millimeter cylinder 

Its bit more precise than the hydrometer .I just tare the cylinder weight off or add it to total then subtract.I wrote the important weights on the wall in studio-I cannot loose them that way. 194 is the glaze I use all the time.

Is there a range of common acceptability in glaze viscosity? I am wondering if there is or not just to get me started as I work at fine tuning the glazes.

 

I ordered a 100ml test tube beaker thing. I think if I understand what you have been saying is that I can use it with the hydrometer as well as for weighing if it seems to be simpler for people. I plan to use those clear pocket sheets for binders for marking the buckets. I can tape them on, slip the sheet with all its information into it. Information such as glaze name, formula, viscosity reading, date mixed etc. with so many different people using the facilities I am trying to come up with a way that sets up a standard for them to use.

 

I really appreciate all your input on this subject.

 

T

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Pug just to be clear-I have a glaze that likes to be about 1520 on the hydrometer scale.

That same glaze weighed up in the 100Ml plastic tube (which also includes the tube weight as its not tare off) is 194 for my optimum best results 

Its fast as I can stir the glaze pour the 100ml full to the line plop it on a scale and adjust it until it weights 194 by adding more water or more material.

It best to always remain on the add water side as it easier to add water than mix and sleeve more glaze to make it heavier.

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Pug asked: "Is there a range of common acceptability in glaze viscosity? I am wondering if there is or not just to get me started as I work at fine tuning the glazes".
 
The quick answer is no.
 
The viscosity (really we should think rheology instead of viscosity) of a glaze is primarily controlled by the ratio of the volume the solids and the volume the water suspending the solids.  There is also the interactions among the solids and between the solids and the water.  The particle size distributions are important, especially the very small and the very large particles.  Some ingredients have low densities and take up lots of room, and others have high densities and don't take as much room for the same weights in a glaze batch.  A very concentrated glaze (high grams/100 ml) may be less viscous than a less concentrated glaze (lower grams/100ml) because there is less interaction among the particles to form flocs or semi-gels.  My gerstley borate glazes have a high viscosity even when the concentration is very low and some of my iron tenmoku glazes are very low viscosity even when highly concentrated.

 

I essentially ignore the 'viscosity' unless it interferes with the fired appearance of the glaze.  Some glazes have special needs, and if they become 'fussy' I usually find another glaze to use.  
 
By trial, error, and patience, I tune each glaze recipe I routinely use to a narrow range of water-to-solids weight ratio by always measuring the amount of water I use for a batch.  When I settle on a ratio, that ratio becomes part of the recipe.  I then measure the specific gravity using the weight of 100 ml of glaze.
 
For a glaze that is new to me I start with a water to solids weight ratio of between 0.6 and 0.7.  I want to start on the concentrated side to begin adjusting because I can just add a bit of water.  Other wise I have to make up more solids to add to the mix.  I NEVER just let it settle and skim off the water!
 
The 'perfect' viscosity(or concentration) depends on how the glaze is applied and the porosity of the bisque ware it is going onto.  When I spray, I dilute a small amount for the sprayer with water.  Most of my work is either dipped or poured, mostly dipped.  I mix my glazes to suit the glaze and the glazing technique.  For one tenmoku glaze I have two buckets, the main bucket with a thicker glaze for dipping and pouring, and a smaller bucket of thin glaze for brushing on a very thin glaze.  The effects are different. The thick glaze completely covers the surface and is opaque, the thin glaze barely covers the pieces and has a transparent rust color with the clay texture dominating the surface.

 

For the real nerdy​ types it is possible to calculate a reasonable target water-to-solid ratio using the true densities of each of the ingredients in the recipe.  To my knowledge, none of the glaze calculation software packages make that calculation. 

 

LT

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Thank you both!

 

So here is my current plan after reading all the responses here as well as some glaze books and even manufacturer glaze sheets:

 

I plan to make up a bunch of small bowls, get the glazes to about where I think they should be using the dip a finger method, use the hydrometer, write that on the bottom of the bowl, fire it, check to see how the glaze has fired, then if needed adjust the glaze, repeat as necessary until I can get a suitable test fire with it, then write that number on the glaze container label.

 

With pottery there never seems to be a one size fits all kind of rule and I know the answer is always TEST TEST TEST.

 

Thanks again!

T

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