How to Make a Floating Stick Hydrometer for the Cost of a Milkshake


Getting glazes to come out the same from batch to batch is a lot easier if you are sure the glaze is mixed to the same consistency each time. Many potters just eyeball it (you know, “mix to the consistency of skim milk”), but to get scientific, you need to measure the specific gravity with a hydrometer.

Making a floating-stick hydrometer doesn’t have to be rocket science. While making calibrating marks on a simple soda straw to show the specific gravity does requires a bit of math, using a calibration chart makes it a lot easier to do. In today’s post, an excerpt from the June/July/August 2013 issue of Ceramics Monthly, Roger Graham shows you how to make and use this simple tool.- Jennifer Harnetty, editor.

 Any Easy and Cheap Way to Make a Hydrometer

by Roger Graham

If you mix your own glazes, you will already have discovered that it is a mixture of science and magic. Probably you make use of an accurate scale for weighing out ingredients. Maybe you consult a glaze calculation program, and work out expansion coefficients. That’s science (with a small “s”). But when it comes to actually mixing the glaze, how much water do you add? Does your recipe say, “Mix to the consistency of thin cream?” Or do you just dip your hand in and see how thick a coating clings to your fingers? That’s Magic (with a capital “M”).

One way to bring glaze consistency assessment back to the realm of science is to use a hydrometer, which measures the specific gravity of a fluid. Let me share with you the design for a glaze hydrometer, which you can make for the price of a milkshake.

Develop your own glazes!
 There’s nothing more satisfying than putting one of your own glazes on a pot you made — and it’s not that hard to do! It doesn’t take much to get started — a half dozen ingredients and a few colorants can keep you busy testing for a year with hundreds of results. Greg Daly’s Developing Glazes provides a great starting point for making your own glazes at any temperature range.

Making the Hydrometer

The idea is to fix the little weight (Figure 1) inside one end of the straw, and seal it in with something waterproof. Then you can float the straw upright in the glaze, and the numbers on the straw will show the specific gravity of the fluid. Floating in pure water, the straw should sink to the mark 1.0, meaning that 1 milliliter of the liquid by volume would be 1 gram by weight. If the straw sinks to the mark 1.4 in your glaze, that means 1 milliliter of glaze weighs 1.4 grams, and so on.

If you don’t have scales suitable for measuring in grams, you can just do it by trial and error. A 20 mm length of ¼-inch steel rod is a good starting point. Just trim it down to about 4 grams using a hacksaw or grinder. A scrap of sheet lead just over a millimeter thick, and about 15 x 20 mm in size, is about right, if that’s what you’ve got. Roll it up into a little cylinder and secure it into the end of the straw with the sealant. Whatever little object you use, it must go completely into the straw (no bits sticking out) and the end of the straw should be closed completely by the sealant. Leave the top of the straw open if you wish, but don’t get liquid inside the straw when in use.

Figure 1

Figure 1

Figure 2

Figure 2

Float the newly made hydrometer straw in water. It should sink a bit over half way, and float upright. Make a mark at the water level (Figure 2).

Now dry the straw and line it up with the calibration chart so the bottom end of the straw is on the bottom line, and the mark you’ve made on the straw is against the chart line marked 1.0 (Figure 3). The sloping cross-lines on the chart show where to mark the other numbers, 1.1, 1.2 , 1.3, 1.4, 1.5, 1.6, 1.7 (Figure 4). A value of 1.3 to 1.5 is typical for most pottery glazes (Figure 5). The exact value probably matters less than being able to get it the same next time, or every time.

Figure 3

Figure 3

If you prefer not to use the calibration chart, it’s easy math to calculate where to put the calibration marks. Just float the newly made straw in water, and mark the water level as 1.0. Now measure the length from the bottom of the straw to the water-level mark, in millimeters. Call that length x. With a calculator, work out a value for (x ⁄ 1.1). That tells you the measurement in millimeters where you should add the calibration mark 1.1. Calculate a value for (x ⁄ 1.2), and mark the straw 1.2, then do it again for (x ⁄ 1.3) and so on.

Figure 4

Figure 4

Figure 5

Figure 5

**First published in June 2013
  • What a great post! I didn’t want to have a fragile glass hydrometer hanging around the house, this sure sounds like it will do the job. I love it when folks give detailed measurements etc instead of generalized info. Thank you.

  • Great post. Thank you so much. I have never had one but this is so easy to make thanks to the clear instructions, I plan to make one.

  • Thank you so much for this great how-to post!
    I’m a novice at ceramics, and sometimes it’s hard for me to decide weather the glaze has good consistency. The hydrometer here costs 30$ at my supply shop..

  • No need for actual measurements, just mark the straw at the level of your current batch, then you can make all next batches the same. Keep different straws for each glaze.

  • I am in agreement with jan-ceramica low. One precaution is that you source the (raw) material for glaze from the same source and of the same quality.

  • I would also like to introduce the science of “Rheology” into this. This is the study of the deformation and flow of matter, the branch of physics concerned with the flow and change of shape of matter. I think it is more scientific than the “Hydrometer”.

    Anyone who has cooked, baked, or played in a sandbox or bubble bath has experimented with rheology. Discussing the deformation of food is a way to introduce the subject of rheology, but the rheology of food is only one subfield of the broad science of rheology. Flows of elastic solutions and of those containing long-chain polymers, including coatings, as well as flows in extruders, molds, and other processing equipment, dominate rheology today. Many industrial problems involve rheological concerns. These include the need to understand the transport of foams and yield-stress fluids in oil drilling and enhanced oil recovery, and the importance of understanding the behavior of biological macromolecules in microfluidic devices for lab-on-a-chip applications. Geoscientists invoke rheology in studies of volcanism and the convection through Earth’s mantle and outer core.

    I agree it is not a household science. But it is much perfect. Any Comments.

  • Great idea!!! thanks!!
    also remember that sq is not the same as viscosity. as noted above different ones for different glazes… i have even tried measure density volume and weight. bottom line is use this to get in ballpark and then test tiles. 😉

  • Looking forward to trying out the hydrometer and again very clear instructions. Thanks for sharing.

  • We have used a hydrometer for years, a peer uses the drops of glaze off his middle finger when dipped to the second knuckle. I may reflect on the appearance of a glaze and the way the hairs on my hand sit in the glaze.

    My Hydrometer is made of PVC and has hacksaw blade knicks in it to help see the marks. We also put marks in it for each glaze.

    Once a glaze goes over SG 1.5, I’ve noticed the rheology if thats what it is or inter-particle behaviours do mess with your head and also make it harder to see the marks on your hydrometer.

    The hydrometer is a pretty fair check system. Des Howard’s straw system is awesome.
    Most potters also own a set of scales, such as a triple beam balance. If you measure a set volume of glaze, like 500ml or 1L then it’s an easy matter to extrapolate the exact SG. Record it on a piece of sticky tape on the side of the glazes bucket, voila, the start of a system.

    Then, observe your glazes as they come out of the kiln, reflect on how thick the glaze was applied and whether it gave you the appropriate result and consider adjusting the glaze thickness or your glazing technique.

    Other things to consider, we glaze small pots quicker than big pots and it’s very important to have a standard bisque process.

    Keep up the good work Ceramic Arts Daily.

  • Does the straw have to be the diameter of the milkshake straw fro McDs or can I use a standard sized straw? Marion

  • Marion

    I just made one using a thinner straw and it took just under 2 grams to make it work.


  • Although this is a cute project, it occurs to me that there is a simpler solution. Assuming that you are mixing your own glazes, you probably have a graduated cylinder. Just do the following:

    1. Use a graduated cylinder which will measure 100cc
    2. Put the cylinder on your scale and tare it for this container.
    3. Put 100cc or glaze in the cylinder and measure the weight. The weight is equal to the specific gravity.

    i.e. 122 grams equals 1.22 sg

  • I do something similar. I have a 60cc syringe. I tare the syringe, draw up 60cc’s of glaze and weigh it. Divide the weight by 60 and there’s my SG. I have used a hydrometer for years but I find weighing is more accurate for me.

  • This will certainly work, and you can interpolate between marks. I made a hydrometer many years age using a weighted wooden stick using the same procedure and it worked great. It was more durable than a straw.

  • I wanted to buy a hydrometer a few weeks ago and thought it would be easy and inexpensive. However, all the cheap ones are aimed at the home brew market and measure with fair precision at the specific gravity that home brewers need … About 1.0 – 1.05. The only ones available here in the UK for the range of SG we need as potters are much more expensive.
    I have been using the weighed container of specific volume method that others have described here, but it is messy… Leaves you with extra cleaning to do. I’m really grateful for this bit of instruction and plan to make one today!!!

    My 2p worth….:-)

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