All Glossary

Specific gravity

In ceramics, the specific gravity of slurries tells us their water-to-solids ratio. That ratio is a key indicator of performance and enabler of consistency.

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A comparison of the weights of equal volumes of a given liquid and water. Water has a specific gravity of 1.0. A ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than water. The specific gravity of ceramic slurries rises as the percentage water content drops. In ceramics the specific gravity of clay, engobe, underglaze and glaze slurries must be monitored.

Measuring Specific Gravity:

• Quick and easy way: Use a hydrometer. Readings are taken on the scale up the side at the liquid surface. The higher the specific gravity, the higher it floats. However viscous slips impede free floating and give an incorrect reading. Hydrometers having too large of a range are not accurate (like reading seconds on the hour hand of a clock). Plenty of slip is needed to have enough depth to float the hydrometer.
• Accurate way: Counterbalance a graduated cylinder and pour in some slip. Divide the weight, in grams, by the volume in millilitres. The bigger the cylinder the more accurate the measurement (e.g. 10cc cylinder is much less accurate than a 100cc one). The cylinder does not need to be filled to a specific mark, the most important is to pour the slip into it with no hanging on to the sides and be able to accurately read its level on the scale.

Specific gravity is very important in the production of casting slips where low-as-possible (or, more accurately, low-as-practical) water content is needed. Hobbyists and potters typically target around 1.75 whereas in industry 1.8 or higher is needed (especially in sanitaryware). Normal clay-based slurries used in traditional ceramics that are outside this range just do not have good working properties. Achieving a slurry having a specific gravity approaching twice that of water is only possible using the deflocculation process. When slurries are not behaving normally (e.g. settling, gelling, casting too slowly or unevenly, not draining from the mold properly, not releasing, producing a powdery surface) the first step in isolating the reason is a specific gravity measurement. Corrections in water content are always made before assessing whether the amount of deflocculant is correct. A good example of the logic is a slurry having too low a specific gravity: It can exhibit similar casting issues to an over-deflocculated one, the best way to know whether a water or deflocculant change is needed is knowing the specific gravity.

Glazes do not have a universally desirable specific gravity range like casting slips. The same glaze can be used effectively by different people and in different processes having quite different specific gravities. We have seen production glazes with specific gravities approaching 1.8. Some industries even prepare their glazes up to 2.0 (obviously highly deflocculated), something that would be impossible for the average potter. Commercial brushing glazes can have specific gravities ranging from below 1.25 up to 1.55 (some companies emphasize brushability and their products are on the lower end). Caution is needed here. Some glaze recipes, when mixed to the apparent correct viscosity (having no additives), will have a fairly high specific gravity (e.g. 1.55-1.6). These commonly settle out into a hard layer on the bottom of the container. Raising the water content (thus lowering the specific gravity) and gelling the slurry (using vinegar for example) is a way to deal with this. Notwithstanding this, doing the opposite, deflocculating the slurry, is also a way to prevent sedimentation (provided it is still viscous). Potters who make their own glaze generally do best with them between 1.45 and 1.50.

Dipping glaze slurries that are 'gelled' work best for potters because they apply more evenly, drip less and do not settle. To be gelled the specific gravity has to be lower-than-normal and then a flocculant added to bring it back to a creamy consistency. For many common partially-fritted glaze recipes this occurs around 1.43 to 1.45 specific gravity. The addition of Epsom salts or vinegar to the slurry gels it and imparts the property of thixotropy. The important thing is to determine, by experience with a specific glaze, what specific gravity and flocculation procedure works well, then stick to that for future batches.

Dark-colored, paint-on glazes used for detail work need to have a high specific gravity so they can cover well with a single brush stroke (e.g. 1.55). Clear glazes that need to be applied (by painting) in a thin layer need a lower specific gravity (e.g. 1.25).

It may be that measuring the water content is easier and more appropriate for what you are doing. It can also be easier to measure if you have an automated device or a heat lamp.

Related Information

Slip having proper rheology is so much better

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This deflocculated slurry of 1.79 specific gravity (only 28% water) has just been poured into a mold. The mold is dry, the wall thickness of the bowl will build quickly and the liquid level will sink only slightly. It can be drained in minutes (for a wall thickness of 3-4 mm). The clay is not too plastic (too fine particle sized) so it is permeable enough to enable efficient water migration to the plastic face. If the specific gravity of this slip was too low (too high a percentage of water) the liquid level would sink drastically during the time in the mold, take longer to build up a wall thickness and water-log the mold quickly. If the slip contained too much deflocculant it would cast slower, settle out, form a skin and drain poorly. If it had too little deflocculant it would gel in the mold and be difficult to pour out. The rheology is just right.

When to use a hydrometer and when not to

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If a glaze has already been mixed and gelled to give it thixotropy these things won't bob up and down to home in on the right level. If the glaze is watery enough there are other issues. The one on the right has a 1.0-1.7 scale. Since most pottery glazes need to be 1.4-1.5 specific gravity (40-50 on this scale) it is difficult to get a very accurate reading. And it is long, you will need a container tall enough to float it and enough glaze to fill it. The small hydrometer appears better, it has a scale of 1.2 to 1.45. But it really bobs up and down (so it is even more important that the slurry be runny and thin to give it the freedom to do so). But glaze slurries are creamy. So it is better to weigh a measured volume of glaze slurry and calculate the SG instead. The easiest is a 100cc graduated cylinder (from Amazon.com), if 100 ccs weighs 140 grams, that is 1.4 specific gravity. You don't even have to pour in 100cc, just pour in any amount and divide the weight by the amount.

More problems measuring glaze specific gravity using a hydrometer

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Potters sometimes call this a "floating thingy". Maybe, because of the problems it presents, it does not make a big enough impression to remember the name! Because of the length of the hydrometer the only container we have is this graduated cylinder. I have to fill it just the right level so it reads near the top. OK, fine. But the hydrometer needs to bob up and down to find home. However, this glaze has our desired thixotropy, which prevents free movement. OK, I will carefully help it find home by pushing it down a little. But then it doesn’t want to bob back up! Ok, I’ll pull it up ... then is doesn't want to go back down to where it should float. Not great. Next problem: The glaze is opaque, I can’t see the reading. Yikes! A better way would be to throw out the hydrometer and just tare the empty cylinder on a scale, fill it to 100 and read the SG as the weight/100. Or fill it to any mark and divide the weight by that.

Carefully measure specific gravity using a scale and graduated cylinder

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This is a convenient way to measure the specific gravity of a glaze or clay slurry fairly accurately. Get a graduated cylinder like this (e.g. Amazon.com). Be careful with cheap plastic ones, check them with water and mark the true 100cc mark if needed. Counterbalance the empty graduated cylinder to zero on your 0.01g scale. Fill it to the 100 mark. The specific gravity is the weight divided by 100.

Measure specific gravity using a scale and measuring cup

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The specific gravity of a suspension is simply its weight compared to water (water weighs 1g per cc). For example, different glazes optimize to different specific gravities (e.g. 1.45 is typical for stoneware glazes). Casting slips typically have specific gravities of 1.75-1.8. This plastic measuring cup accommodates 500g. I have already counter-weighed it on our 0.1g scale and filled it with 500 grams of water to verify the accuracy of the 500g marking (mark the correct level with a felt pen or sticker if needed). Now, I am filling the cup with the slurry to the same level. The specific gravity is this weight divided by 500.

Quickly measure specific gravity using a scale and graduated cylinder

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Counterbalance a graduated cylinder on a 0.01g scale and pour in some slurry (being careful not to exceed the total weight the scale can handle). Fill it to any level. Divide the weight by the volume. In this case, it weighs 60.6g and the volume is 41. That calculates to about 1.47 specific gravity. The higher it is filled the more accurate the measure will be.

A casting slip of 1.9 specific gravity. Should we use it?

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A hydrometer is being used to check the specific gravity of a ceramic casting slip in a graduated cylinder. Common traditional clay-containing ceramic slips are usually maintained around 1.75-1.8. In this case the slurry was too heavy, almost 1.9. Yet it is very fluid, why is this? It has both too much clay and too much deflocculant. While it is possible to use such a slip, it will not drain as well and it will gel too quickly as it stands. It is better to settle for a lower specific gravity (where you can control the thixotropy and it is easier to use). It might have been better to simply fill a 100cc cylinder and weigh it to get the specific gravity (slurries that are very viscous do not permit hydrometers to float freely).

What to do when glazes drain and drip like this

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These are the same dipping glaze, it is variegated and drips look ugly on fired ware. The slurry of the one on the left had a specific gravity of 1.45, it was creamy and appeared to be good. However, when this bisque porcelain mug was pulled out after the dip and rolled it formed drips on the rim. Why fix the slurry instead of just rubbing this out? Keep reading. To fix this I added water to decrease the specific gravity to 1.43. Then I added Epsom salts to induce thixotropy and gel it back to a creamy consistency. This time it went on evenly. Although the piece dries enough to handle in less than 30 seconds, it does take longer to dry completely because there is more water in the slurry. Adding CMC gum would also slow down drying but it would be very tricky to get just the right amount (otherwise dripping could be worse than this). Consider a few of the advantages of thixotropic dipping glazes: They don't settle like a rock in the bucket, they don't go on too thick or in multiple thicknesses, they don't drip and they don't dry too fast.

Glaze at 1.7 specific gravity on green-ware. Way too thick!

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This is G2926B clear cone 6 glaze deflocculated with Darvan. Because the Darvan is thinning it, 2.5kg of glaze powder is suspended in only 1100g (1100ml) of water (half the normal amount). While the slurry in the bucket flows well and appears like it should work, a one-second dip produces twice the desired thickness. It dries slowly and it is very difficult to prevent runs. The lesson: Make sure the specific gravity (SG) of your glazes is right. What should the SG be? Measure it when your glaze is working well. Or take note of it in instructions that come with the recipes you use. For bisque ware: 1.43-1.45 with a flocculant (like Vinegar or powdered Epsom Salts) added to gel the slurry slightly.

Specific gravities on three commercial glazes might surprise you

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The freshly opened transparent low-fire glaze on the left has a specific gravity of only 1.34 (a high water content compared to dipping glazes). Yet it is viscous and holds in place because they add a lot of gum. It needs three coats to go on thick enough and requires quite a bit of time to dry each one. Highly fritted transparent glazes need to be applied thin enough to be clear, thick enough to be glassy smooth but not too thick (to avoid clouding). Amaco likely targets the low specific gravity to enable good control of thickness. The center Potter's Choice glaze, is 1.52. And thus goes on nice and thick with each coat. That glaze likely contains lots of clay so little or no gelling agent (e.g. Veegum) is needed. The Celadon glaze on the right is in between, 1.46. Glaze manufacturers can produce at a broad range of specific gravities, they adapt the percentage of gum (e.g. Veegum and CMC gum) to impart the needed rheology and brushing characteristics.

Very low specific gravities on two commercial underglazes

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AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water)! The former is well below their recommended specific gravity of 1.4 (it still paints well but needs more coats and more time to dry and apply them). The Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (so it likely contains more gelling agent). When doing underglaze decorative brushwork it is important to get adequate thickness with each brush stroke, so a higher specific gravity is better. This may be reason enough to consider making your own (by adding stain powders to a base and using Veegum CER to gel the slurry, slow down its drying and harden it well at the dried state).

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The glaze firing reveals that the specific gravity of this V-303 terra cotta under glaze is too low

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