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Epsom Salts


Formula: MgSO4.7H2O
Alternate Names: Magnesium Sulfate, HEPTAHYDRATE, hydrous magnesium sulphate

Oxide Weight40.30
Formula Weight246.48
If this formula is not unified correctly please contact us.
DENS - Density (Specific Gravity) 1.68

Colorless transparent crystals. Also known as magnesium sulfate.

Magnesium sulfate most commonly employed in ceramics as a flocculant. It thickens and gels a glaze or engobe suspension by electrostatically charging particles so that they attract each other more. This not only suspends them in the slurry but makes them adhere to non-porous surfaces without running off (by virtue of the gelling mechanism). Clay particles are not needed, other materials will also flocculate.

Epsom salts is generally better for flocculating ceramic glaze suspensions than vinegar (although less easily obtained). Calcium chloride is better than both (and the least easily obtained).

The addition of Epsom salts is commonly done with engobes. A gelled engobe will apply in a even coat and stay in place even though it takes time to dry. Without the Epsom salt addition, the engobe will run and drip (even though it would have a lower water content). Since higher water content is implicit with epsom-salt-conditioned-slurries, they do shrink more on drying and this could cause cracking if the clay content is too high or the clays present are too plastic.

Epsom salt additions are also common where a thin yet even layer of glaze is required, such as with low fire transparent glazed earthenware (these often cloud or blister if applied too thick, yet produce a brilliantly glossy surface where thin).

The most effective addition strategy is to make a saturated solution and add this in very small amounts to a slurry. If the crystals are added directly it takes time for them to dissolve and act and it is very easy to overdo it and thicken the slurry too much (if you do not give the crystals time to dissolve). Notwithstanding this, if you have a good mixer than can run constantly, sprinkling in the powder and waiting thirty seconds for the reaction is not impractical (and you do not have to fiddle with making the solution).

Usually only about 0.1% is needed, but up to 0.5% can be used with particularly troublesome slurries.

Some clay body manufacturers add Epsom salts to their clay mixes to improve plasticity and stabilize bodies against the thixotropic and spontaneous softening effects of certain soluble compounds in the mix (e.g. soda feldspar, nepheline syenite). It is typical to use .2-.3%.

It is sometimes said that epsom salts can be a helpful addition to glazes containing Gerstley Borate to help prevent particle agglomeration of a slurry that causes it to gel (try about 4 g per 100g of Gerstley Borate). However, this is not our experience; Gerstley Borate gels slurries on its own and deflocculation seems like a more logical approach.


The same engobe. Same water content. What is the difference?

The same engobe. Same water content. What is the difference?

The engobe on the left, even though it has a fairly low water content, is running off the leather hard clay, dripping and drying slowly. The one on the right has been flocculated with epsom salts, giving it thixotropy (ability to gel when not in motion but flow when in motion). Now there are no drips, there are no thin or thick sections. It gels after a few seconds and can be uprighted and set on the shelf for drying.

When to use vinegar and when to use epsom salts to flocculate a slurry

When to use vinegar and when to use epsom salts to flocculate a slurry

Slurries with more clay (like engobes, slips) generally respond better to epsom salts. However the extra clay also makes them more likely to go moldy, so you may need to add a few drops of Dettol to kill the bacteria (if they are stored for any length of time). Vinegar works better for glaze surries, but only if they have sufficient specific gravity. Many people like to make an epsom salts solution and add that, but if you have a good mixer you may find it more intuitive to add the crystals and wait 30 seconds for the viscosity to respond.

Feldspar applied as a glaze? Yes! The way I did it will change how you glaze.

Feldspar applied as a glaze? Yes! The way I did it will change how you glaze.

Custer feldspar and Nepheline Syenite. The coverage is perfectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two powders in water until the flow was like heavy cream. I added more water to thin and started adding the epsom salts. After only a pinch or two they both gelled. Then I added more water and more epsom salts until they thickened again and gelled even better. They both applied beautifully to these porcelains. The gelled consistency prevented them settling in seconds to a hard layer on the bucket bottom. Could you do this with pure silica? Yes! The lesson: If these will suspend by gelling with epsom salts then any glaze will. You never need to tolerate settling or uneven coverage again! Read the page "Thixotropy", it will change your life as a potter.

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By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="Epsom Salts" descrip="Flocculant" searchkey="Magnesium Sulfate, HEPTAHYDRATE, hydrous magnesium sulphate" loi="0.00" casnumber="7487-88-9"> <oxides> <oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="" percent="16.360" tolerance=""/> </oxides> <volatiles> <volatile symbol="LOI" name="Loss on Ignition" percent="51.140" tolerance=""/> <volatile symbol="SO3" name="Sulfur Trioxide" percent="32.510" tolerance=""/> </volatiles> </material>

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