Scum is the appearance of deposits on the edges of ceramic vessels and occasionally on the outer surfaces of dry and sintered bodies. In fact, these sediments have already appeared during the drying process of the green body, but it is difficult to be found at that stage. Since the sediment can be melted in the process of biscuit firing, the part of the green body with scum attached is usually more vitrified than other parts, and may react tenderly with the glaze. The scum is usually off-white or light yellow, especially when the clay used for ceramic products is red clay, the color of the scum is brighter. The composition of scum is usually dominated by soluble mixtures such as calcium, sodium, potassium, magnesium, iron and calcium sulfate.
Almost all ceramic blanks contain soluble salts, and the content is very small, so basically it will not cause any problems.
①In the process of preparing ceramic blanks by mud storage method, about half of the soluble salts will be elutriated, but in the later mud refining process, because there is no previous elutriation step, the scum will form A .
②During the drying process, the humidity gradient mainly appears on the edge and mouth of the ceramic vessel. Since the above-mentioned parts can be exposed to the flowing air, the drying speed is faster than other parts on the green body. It can be seen that soluble substances tend to adhere to parts that are more in contact with the air, and their drying rate is relatively high, so they must be treated with caution.
B Slowing down the drying speed of the green body can prevent the appearance of scum. The principle is that slow drying will make the scum evenly dispersed, so that it will not all settle on the edge of the container.
4 reasons and 5 solutions for scum in ceramic water cups
C In the mud refining stage, add 0.5% to 2.5% barium carbonate to the ceramic bad material, the resulting chemical metathesis reaction can effectively prevent the appearance of scum, the reason is that soluble salts will be transformed into insoluble carbon salt. Due to the high value of barium carbonate, the best way to solve the scum problem is to replace the ceramic billet—choose mud with a lower content of soluble salts to make the billet.
③ Some natural raw materials such as ochre have a very high content of soluble salts inside. Therefore, when a large amount of ochre is mixed in the ceramic blank, it is best to add some barium carbonate to the formula to avoid scum . Clay with high content of soluble salts also includes red clay. Most red clay is made from marl, which is rich in lime.
④ In rare cases, fingerprints will appear on the outer surface of the biscuit body. This is because the green body is held and moved before being fired in the kiln, and the contact of human hands with the green body leads to the precipitation of salt substances.
D Immersing the ceramic body in water can promote the discharge of soluble salts in the mud. From my experience, only the water in the static water tank has the above effect. The water vapor discharged from the static water tank can condense the soluble salts in the ceramic body until a scum appears on the outer surface of the vessel, and the static water tank needs to be poured periodically. Sometimes, adding some vinegar (acetic acid) to the water (using a sponge and other tools) during the hydration session can get twice the result with half the effort.
ESometimes, for daily-use ceramics such as mugs and high-swollen cups, letting the body dry can avoid scum. Doing so will deposit the salt on the bottom of the vessel instead of the rim.