Although the EU have downgraded Barium Carbonate from "Poisonous" to "Harmful If Swallowed", it is still treated as a toxic material in various countries across the world. However, its position as the most useful additive for the elimination of salt scumming from clay bricks and tiles is unchanged. For those countries where barium is prohibited, or due to increases in price and reduced availability, manufacturers look for alternatives.
WHAT IS SCUM?
Firstly, I think we need to clarify what scum or scumming is. The unsightly scum is a surface deposit found on clay bricks and tiles. It appears immediately after firing, and preventing it is important.
There are two types of scum:
- Dryer scum.
Scum formed during the drying process. Soluble salts within the raw materials are carried to the surface as the water is being driven off. It can also be formed by sulfurous gasses within the dryer atmosphere forming soluble salts. Usually, these types of salts from drying do not become apparent until after firing.
- Kiln scum.
Scum formed during the firing process in a similar manner to the above, though the primary cause is usually by sulfurous gasses in the kiln atmosphere.
Scumming is caused, therefore, by soluble salts either present in the raw materials or formed during the production process. In most cases, scumming comes from soluble sulfates (especially those of calcium, sodium, potassium and magnesium of which calcium is the most common). All these salts will give a whitish scum which is unsightly on red or dark colored bodies
One of the main methods to combat scumming is the use of additives to the clay mix whereby they use their chemical or physical properties to remove or prevent scumming.
Usually, these additives work in one of four different ways:
- Rendering the soluble salts insoluble by direct precipitation (the most common process within the industry)
- Competing with other constituents for sulfurous gasses and forming insoluble sulfates
- Assisting scum decomposition at low temperatures, hence giving a clear scum
- Changing physical properties making the salts less likely to appear on the surface.
It is worth noting that the effect of any additive is not always consistent as it is governed by many reasons which include:
- The composition of the raw materials themselves, apart from the soluble sulfates, as some clays react well to anti-scumming additives while others do not. Sodium Chloride can render certain additives less useful.
- The preparation process/molding in that those using little water may give less scumming compared to methods which use more water - even with the same clays. Also, the contact time of the additive can be critical, the longer being the better
- Firing and drying conditions. Faster drying is commonly more efficient than slower. Hard firing or reduced firing conditions will tend to destroy any scum formed. Also, the type of kiln and fuel used are relevant factors.
The above factors are important whether clay additives are used or not, but they all may contribute; while one additive may be useful under certain conditions, there is no all-embracing additive.
ALTERNATIVES TO BARIUM CARBONATE
So, what additives, other than barium carbonate, are available?
- Barium chloride
- Sodium based compounds
- Calcium based compounds
- Ammonium based compounds
- Numerous clay conditioner blends and biopolymers
Considering these factors the use of clay conditioner blends/biopolymers have been proven to be able to replace sulphate containing clays, reduce water demand and as such speed up drying times (amongst a number of other benefits) with a lower addition rate than barium carbonate and in some instances at a much lower unit cost.
HOW TO CHOOSE THE RIGHT ADDITIVE
The elimination of scumming is not an exact science, and as discussed, sometimes even the use of Barium Carbonate cannot be relied upon as 100% efficient. Therefore, when you consider replacing Barium Carbonate, either ask your supplier or your internal laboratory to conduct an analysis of your clays for total sulfate levels. Based upon these values your supplier should be able to give a recommendation of products best suitable for you. Dependent upon the product being used it may not only eliminate scumming but also improve production output, yields and reduce costs for you.