Why I Don’t Use a Hydrometer to Measure Specific Gravity

Adding Water to Glazes

I didn’t always know about measuring specific gravity. Of the 10 years that I’ve been mixing glazes, I’ve only been measuring specific gravity for 3 of them. It wasn’t a technique I learned in school.

But… I had heard about it enough times that eventually I used it to try to solve a problem. And eventually (but not right away) it worked!

As the studio technician at a busy ceramics studio, I’m in charge of mixing and maintaining about 20 large buckets of glaze. We have a rule that no one but the technician is allowed to add water to the glazes so it was my responsibility to keep the water at the right level.

People would often let me know that a glaze seemed too thick or thin and I would decide whether I thought it needed more water, based on how it appeared in the bucket. 

I soon realized that thick and thin are subjective terms and people have different definitions of how a glaze “should be”. I wished there was a way to calculate how much water to add so I didn’t have to make a judgement call every time. 

I wanted a measuring device that would take the opinion out of the equation. I started reading up on specific gravity and figured this might be the answer.

 

The Hydrometer

In the studio, I found a tool called a hydrometer. I had never tried it and I don’t think anyone had used it in years. I had a winemaking hydrometer at home but it didn’t have the right scale for glazes. Alcohol is lighter than water. Glazes are heavier.

This new hydrometer said Laguna Clay Co. right on it. Cool, this must be the answer! I dusted it off and was excited to try it out.

Laguna hydrometer on top, wine making hydrometer on bottom.

I had read that you could check the specific gravity (SG) by dropping the hydrometer into the bucket of glaze to see how far it sank into the liquid. There were numbers on the tube and whichever number appeared at the glaze level when the hydrometer was floating in the bucket was your Specific Gravity number.

The numbers ranged from 1.0 – 2.0. The lower the hydrometer sinks into the glaze, the lower the SG reading.

Great! I finally had a system that took the decision making process out of adding water to our glazes. I got all the glazes to where I thought they should be thickness-wise, dropped the hydrometer into each one, took a reading and wrote it on the bucket.

Then, when someone would ask me to check on a glaze, I would grab the hydrometer, mix the glaze, drop it in the bucket, compare the reading to what was written on the bucket, and see if the glaze needed water.

 

Questionable Accuracy

What I learned fairly quickly, was that the hydrometer gave me very inconsistent readings. Sometimes, the same bucket of glaze would give a different reading each time I stuck the hydrometer in. It didn’t make sense.

Specific gravity, by definition, is the ratio of the density of a substance to the density of a reference substance.

In our case, the substance is glaze, and the reference substance is water. So, the specific gravity of a glaze = the ratio of the density of a glaze to the density of water.

So how do we get the density of a glaze?

Density, by definition, is the measurement of mass (g) per unit of volume (mL). 

Science tells us that the density of water is 1 g/mL. To get the SG of a glaze, you take the density of the glaze and divide by the density of water, which is 1 (and easy math!). 

The values of density and specific gravity are equal and can be used interchangeably. So if a glaze has a density of 1.5 g/mL, the specific gravity is 1.5 (easy math!).

The only 2 factors that affect density are mass and volume. So the specific gravity can only change if the mass or the volume changes. If neither the mass, nor the volume has changed, the SG remains the same.

When you add water to a glaze, you are bringing the density down, and closer to 1.

Since I was getting a different hydrometer reading from the exact same bucket of glaze, without changing the mass or the volume, I lost trust in my new system. 

Now, there’s a perfectly good explanation for why a perfectly good hydrometer would give such inaccurate readings. And that explanation is VISCOSITY.

 

Viscosity Tricks the Hydrometer

Viscosity, by definition, is the measurement of a fluid’s resistance to flow due to internal friction.

Some fluids, like water, flow very easily. Water has a relatively low viscosity. 

Other fluids like honey and syrup flow much more slowly due to higher internal friction and resistance within the fluid. These fluids have a much higher viscosity than water. 

Essentially, viscosity refers to the physical thickness of a fluid.

Fun fact: The branch of physics that studies the flow of matter is called Rheology.

When you think of water vs honey, you would probably say the water is thin and the honey is thick. What you are assessing, is their relative viscosity. 

There are several factors that can affect the viscosity of a glaze without changing the water content/density/specific gravity. And viscosity is what throws off the hydrometer reading.

Example: You can have a “thick” glaze with the exact same specific gravity as a “thin” glaze. In a thin glaze, the hydrometer will sink further down, giving a lower reading than in a thick glaze. The hydrometer is affected by the fluid’s viscosity, not its specific gravity.

Another example: You can have a thin glaze and add a flocculant like Epsom Salts to it, making it thicker. You haven’t changed the water content or the density but you’ve increased the viscosity. The hydrometer will give a lower reading before adding the flocculant and a higher reading after adding the flocculant. Same bucket of glaze, 2 different hydrometer readings.

Another example: A glaze containing gerstley borate becomes thixotropic. This means the glaze thickens when it’s sitting still, but when it’s mixed or agitated the viscosity drops and the glaze becomes much thinner. Same glaze, same specific gravity but different viscosity and different hydrometer reading when the glaze is moving vs sitting still.

This photo shows 2 different glazes with the same specific gravity. The one on the left is much thicker (higher viscosity) than the one on the right, even though they both have the same amount of water.

 

In Conclusion…

Given the 3 examples above, it’s clear that a hydrometer is a tool that can be used for assessing the viscosity of a glaze, but not necessarily the specific gravity.

On page 66 of the 1951 book “Ceramic Glazes” by Cullen Parmelee, this is what it says about the use of a hydrometer as a method for measuring specific gravity. 

“The objections to the use of this method are many. The method is applicable only in liquids, whereas the [glaze] is not a solution but a suspension of colloids and crystalloids in a liquid; the viscous properties of the [glaze] falsify the readings of the instrument.”

The hydrometer wasn’t accurate in the 1950s and unfortunately, it’s not accurate today either.

You may still find the hydrometer to be a useful tool if you just want to be able to tell if the viscosity of your glaze has changed since the last time you measured. But to be accurate, I would refer to these measurements as hydrometer readings and not as specific gravity measurements. 

To get an accurate specific gravity reading, you need 2 measurements – mass and volume. So you need 2 tools – a scale and a container. And that’s all it takes to find out exactly how much water is in your glaze so you can make adjustments from there.

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Free Guide!

If you haven’t seen it yet, I created a free, full colour, step by step guide to measuring specific gravity for repeatable glaze results. It includes a one page summary that you can print out and take into your studio. I hope you find it useful!

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