陶文、Show

 
20030625賀小姐:因為我開了一桶釉,沉積得很利害,朋友介紹我加少量Epsom Salts可制止.......

Magnesium Sulfate (Epsom salts)

Some clay suppliers add hydrous magnesium sulfate (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. from soda feldspar, nepheline syenite). It is typical to use .2-.3%.

Magnesium sulfate is also used to 'set' (flocculate, thicken, gel) glazes to suspend them and make them adhere to non-porous surfaces without running off. It forms a mild sulfuric acid that changes the electrostatic charge on clay particles causing them to reorient at right angles to each other. Thus it is typically added to glazes that have adequate clay particles for it to interact with.

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.

Usually only about 0.1% is needed, but up to 0.5% can be used with particularly troublesome glazes. When evaluating how much is needed in a glaze slurry, be careful to give the added material time to dissolve.

Epsom salts can be a helpful addition to glazes containing Gerstley Borate to help prevent particle agglomeration and of a slurry that causes it to gel (try about 4 g per 100g of Gerstley Borate).

Sodium Silicate

The most popular deflocculant used in casting slips for many years. It is nearly always used with soda ash as used alone it makes the slip 'stringy' and thixotropic. The material is effective, reliable and inexpensive. However, it attacks the plaster in molds much more than more modern deflocculants and it is much easier to overdeflocculate a slip with sodium silicate.

There are potassium based deflocculants which are similar to the corresponding sodium ones. They can be employed where the presence of sodium is undesirable. Soda ash is more suitable for deflocculating glazes.

In addition, a wide range of organic deflocculants are available as alkali salts of pyrogallic, humic or tannic acids. They have long working ranges and can increase the apparent plasticity of the clay. Their use prolongs mold life compared with sodium silicate and soda ash. They are less prone of over deflocculation also.

Tetramethylammoniumhydroxide is also an organic compound that is used where residues from inorganic salts cannot be tolerated. It is a strong base that can even attack glass and quartz.