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Bioman

Specific Gravity

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I am new to using a hydrometer to measure S.G..  Is there a starting point reading I can use such as 50 and work from there?  Or will this be one of those "it depends" answers:)  As i did not have a hydrometer when I made up many glaze batches, I have no baseline to start from.

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You don't need a hydrometer. If you have a graduated cylinder, you can put that cylinder on a scale and balance it to 0 grams. Then pour in 100ML of your glaze and it will have a weight in grams, divide by 100 and you have your specific gravity.

 

So say you weight 100ML of glaze and its 162, that means its 1.62 specific gravity. 

 

edit: i misread your question. If you didn't take an approximate when you got the glaze to the consistancy that you wanted you really just have to get the next batch close to the consistency you remember, measure, do a test firing and see if its close, if it is then record that SG.

 

If you still have some glaze that you have been using, but not enough for the hydro meter to fit in, just do the above method and make your next glaze to that same SG.

 

Does that help? I am not sure if I am reading everything correct.

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I don't currently have a graduated cylinder.  The hydrometer I bought has so many scales it is confusing.  There is a lower dual scale that reads 0 - 140 for temperatures and right next to this is one that reads 0 - 150 with no value.  Higher up in the narrow portion of the cylinder is another scale that states it is for "heavy liquids" and reads 10 ( I think they mean 100) to 10.  Any idea how to read one of these?  I'll try to get a graduated, but for now would like to get going.

The glaze is a Mayco dry glaze if that helps at all.

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In a pinch, you can also calibrate a clear plastic cup to your scale by weighing out 100g of water and marking the bottom of the meniscus with an indelible marker.

 

Sorry, it's an "it depends" answer.  Get your glaze to the right consistency by "feel," measure the s.g. and record it! You can do a line blend by adding water to glaze and figuring out exactly what number works the best, rather than stabbing in the dark over multiple firings.  Some glazes, like proper celadons like going on thick. I have one that liked being around 1.45-1.48.  I have recently discovered that this level is waaay too thick for my cone 6 clear, which prefers to be around 1.28-1.3.

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OK, so i found a measuring cup with ml marking.  I weighed out 100 ml of liquid after zeroing as indicated.  The reading I got was 126 gms.  Most of my glazes are Mayco Dry forumals for dipping stoneware.  

The mixing directions which i would have followed are ;  1 lb to 16 oz of water.  Would this result in a reading of 160 gms or 1.6 on a hydrometer or none of the above?

 

PS, does adding more water raise or lower the number I would see? 

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Joseph said "You don't need a hydrometer." I'll take it a step further and say you don't want one either.

 

First, about hydrometers generally - they measure the density of a liquid based on buoyancy, i.e., by how deep in the liquid it floats. In a less dense liquid it will sink deeper, and in in a more dense liquid it will float higher. The scale marked on the side of the glass tube is there for reference (and repeatability). Unfortunately, there are several different scales for different purposes. One of the scales is intended for liquids that are less dense than water, such as alcohol. Hydrometers with this scale are typically used by beer and wine makers. Another scale is intended for liquids more dense than water, such as water containing dissolved or suspended things. These scales are do not give you the same number result for the same liquid. Thus if someone with a light beer hydrometer measures a glaze slurry and tells you the number, and you have a heavy hydrometer and use that number to adjust your glaze, it's going to be wrong. From your description, your hydrometer has multiple scales marked on it, but you still need to know which scale was the one used by the other person before your glaze will be the same.

 

Furthermore, the scale does not equate to specific gravity. A hydrometer measures in degrees Baume. (Google it for a more complete explanation.) Yes, you can do some math to convert degrees Baume to physical specific gravity, but now you've added a layer of complication. Particularly if someone who has no idea of the differences tells you the "specific gravity" of a glaze is xyz, as measured with their handy dandy hydrometer. Not quite as absurd as reporting humidity as measured with a barometer, but you get the idea.

 

Finally, a hydrometer, even if you understand all the nuances of the scaling, is notoriously inaccurate for measuring the density/specific gravity of glaze slurries because of the thixotropic characteristics of glaze slurries. The "friction" the glaze slurry exerts on the sides of the hydrometer will cause the hydrometer to come to rest somewhere other than where it's proper buoyancy should leave it. Particularly thixotropic glaze slurries, such as those high in Gerstley Borate or Neph Sye, change their apparent hydrometer levels depending on how recently the slurry has been vigorously stirred. For some glazes, that can be near instantaneous; by the time the stirred slurry stops swirling in the bucket enough that you can drop the hydrometer in without it being smashed on the side of the bucket or being pulled under, it is already thickening up and the hydrometer will sit too high.

 

So, where does one go now? As suggested, true specific gravity is the ratio of the weight of the glaze to the weight of an equal amount of pure water. Some have suggested a 100ml beaker. Another possibility is a veterinary syringe of 10 or 100ml. You might find something appropriate in the drugstore. The point is find something that holds an exact 10 or 100ml of liquid. 10ml of water weighs exactly 10 grams, and 100ml = 100g. This simply makes the math easy. A container measuring some other amount requires calculator math to derive the ratio. Now, put your  empty10/100ml container on the digital scale and tare/zero it to take the weight of the container out of play. Fill the container with exactly 10 or 100 ml of glaze and weigh it again. The weight of the glaze will convert to specific gravity just by shifting the decimal point. 15.2 grams of 10 ml of glaze is a S.G. of 152. If you are using a 100ml container, the weight of the glaze in grams is the S.G. without shifting the decimal.

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Ok, some followup to your latest questions which you put up while I was writing. Typical dipping glazes are in the 150-160 S.G. range. A lower number is a thinner glaze slurry, until you reach 100, which is straight water with no glaze in it. A higher number is more glaze solids and proportionately less water. So, if you want to lower the S.G., you must add water, a bit at a time and measure again until you have adjusted it to where you want it. If you want to raise the S.G. to a higher number, you have to take some water out, which is not easily accomplished. You can let the glaze slurry sit and settle for a day or so and then scoop off some of the clear water on top (sometimes you can use a big sponge gently laid on top the water to sop some up) or leave the bucket sit open for a few days and let some water evaporate.

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Another thing about what Dick said is that some glazes are generally thicker/thinner than others. So you might have a lower SG than other glazes and be the same exact consistency in the bucket as a glaze with a higher SG.  It all depends really and trying to find a general number for all glazes is a waste of time. Find the consistency you like for the glaze your using, record it, then next time you mix a batch if your using the same materials, aim for that same SG. 

 

I dont even fool with SG 95% of the time. I just do the finger dip test. If I can dip my finger in the glaze and a tiny bead forms on the end but doesn't drip, the glaze is about right my uses.

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Bioman

I've have used a hydrometer forever

I just went to studio so I could give you the right info.

They make them is many scales-some for beer making some for slip making and some for glaze making-some for whatever .

The one you want is called (and this is printed on paper inside the glass tube) Specific Gravity-Baume Heavy-temp,60/60F USA made

It sounds like you have some other type .Do not spend the time fuzzing with the wrong ones as these get broken over time so buy a few of the right ones.

On the right type there are two scales 2000 up to1000 with a number every 100  graduated in 10ths (this is the scale I use 

the other scale is70 up to zero every 10 marked with graduations between in 10'ths-its bait less accurate .

I use the 100 ml graduated cylinder now for all critical measures as to water amount as its more accurate 

Especially my rutile blue base and a two other glazes I use.

I can tell but my hand with a latex glove on the right amount but the weights is the best way when it matters say on a 35 cubic foot kiln of rutile base glaze.

I would get the right hydrometer (from a ceramic supply) as well as the plastic graduated cylinder and start from there.

 

One last note I use my hydrometer wet meaning I dip it in water before measuring glaze-you need to decide wet or dry as they measurements will be different -if you chose wet you will not have to dry it every time. Small point but missed by many.It's as many things in ceramics the devil is in the details and this is one.

I learned to use these tools later in my ceramic career as the hand technique does 99% of most glazes

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I have never used one. I running the ceramics lab in the University for 25 years, I had 12 or so shop glazes and they were all mixed to the same consistency. Not fickle glazes.Stoneware and then cone6 reduction.

Majolica was mixed a tad thicker.

Pots were washed lightly before applying the glazes.

The glazes went on to the right consistency.

I still use the similar bases in my cone 6 ox. glazes.

I guess I learned in a low tech environment.I used the knuckle wrinkles method i.e the lines in the knuckle show after dipping a dry finger into the glaze.

Marcia

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So far I have not had any issues with my wares coming out of the kiln that could not be attributed to firing profiles which corrected pin-holing and such.  Being new I was simply doing the glaze S.G. thing because I had read it was important and thought perhaps I am missing something. 

 

To date I have kept by glazes in buckets with Gamma seals (O-ringed), so any significant evaporation is not occurring due to leaky lids. 

 

I have a question.  Does the very act of dipping bisque change the water/solids ratio over time or is it more an issue of evaporation due to storage that does this and or both?

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So far I have not had any issues with my wares coming out of the kiln that could not be attributed to firing profiles which corrected pin-holing and such.  Being new I was simply doing the glaze S.G. thing because I had read it was important and thought perhaps I am missing something. 

 

To date I have kept by glazes in buckets with Gamma seals (O-ringed), so any significant evaporation is not occurring due to leaky lids. 

 

I have a question.  Does the very act of dipping bisque change the water/solids ratio over time or is it more an issue of evaporation due to storage that does this and or both?

There is another way to find out if your glaze  has the right consistency and that is to use a spatula, stir the glaze and then at about a 30 deg angle let the glaze roll off and count the drips.  So if it runs off like water it is too thin, if it runs off than drips about 6 to 7 drips, at least for me it is fine.  Yes the consistency will change as you are glazing pots.

David

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So far I have not had any issues with my wares coming out of the kiln that could not be attributed to firing profiles which corrected pin-holing and such.  Being new I was simply doing the glaze S.G. thing because I had read it was important and thought perhaps I am missing something. 

 

To date I have kept by glazes in buckets with Gamma seals (O-ringed), so any significant evaporation is not occurring due to leaky lids. 

 

I have a question.  Does the very act of dipping bisque change the water/solids ratio over time or is it more an issue of evaporation due to storage that does this and or both?

So if it runs off like water it is too thin, if it runs off than drips about 6 to 7 drips, at least for me it is fine.  

David

 

 

See this is complete opposite of me. I like my glazes absurdly thick maybe 1-2 drips at most.  This is a good example of why SG is kinda useless in general until you figure out what you like your glazes like. 

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but isnt consistency related to glaze application? 

 

is the basic assumption here that you dip your glazes? 

 

i know if i dip my glazes too long they either peel off while drying or they shiver. so i thought if the glazes were too thick they would cause problems.

 

i know every school has different rules. 

 

i watched warren mackenzie quickly dip his pot in two glazes. and i tried that since our glazes were as runny as his - like milk. i went against our 2 alligator dip rule and followed him and of course the 2 alligator made the best colour. of course this question entered my mind because i saw a visiting korean potter have two different consistencies of slip (buncheong ware) for dipping (milk) and brushing (yoghurt). 

 

at our school studio i have used the SG tube. the glazes had  different numbers depending on its composition. 

 

so if you want food safe ware by western standards, there has to be each food area covered with glaze right?

 

but in the end i guess as mark said test, test, test.

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To my mind, if it is just for your personal use, the scale on the hydrometer doesn't matter - you just need to know that glaze x works best with a reading of y.

But hydrometers have problems, not just with the thixotropicity of the glaze, but also if you have a glaze that settles quickly and is not well stirred  then the top of the glaze is more watery, and so lower SG, than the bottom as it begins to settle.

Personally I just use a 10ml syringe - cheaply bought on eBay, or ask your friendly vet/drug dealer.

 

Another thing people haven't mentioned is that SG is only half the story, you also need to consider viscosity. Those who do slip casting will know all about this, but basically adding a deflocculant like sodium silicate means that for a given density your glaze will be more fluid, whereas adding a flocculant like vinegar makes the glaze thicker and also helps keep it in suspension. Apart from helping ensure the glaze stays suspended, I've also recently being experimenting with deflocculated slip for applying to green ware, after some bowls collapsed from too much water being absorbed after they were dipped in the slip. Also, if you have a newly mixed glaze you will often find it is initially quite runny and then thickens over the next few days, as the clay particles break up and mix in with the water more. You can get a small and cheap viscosity cup for paint to measure this - basically it has a hole in the bottom and you time it to empty out.

 

So ideally you want to measure both the SG and viscosity, then adjust to get the SG right, then add flocculant/deflocculant to adjust viscosity.

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I have swineherd over to the 100ml graduated weigh its easier to and faster to do

But as Tim said above ( you just need to know that glaze x works best with a reading of y.)

if you chose to use a hydrometer you mix the glaze well and as soon as possible (when the surface is not spinning fast) take your measurement-you need to always do this as he said settling will affect the measurement.

You can use a ford cup as well (I have a few and have used them as well)

 

I like the weighed 100ml plastic cylinder-I use it each week on my one  glaze that is so touchy (rule blue) -all the rest of the 12-15 buckets are by feel.

When you glaze/and fire each week you get a good feel for whats going on.

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