MCA tiles DO NOT have Black Core!

Black Coring in Roof Tiles-Effects on Properties and Performance
By Dr. Vernon L. Burdick, Professor of Ceramic Engineering
New York State College of Ceramics at Alfred University

Black coring in clay roof tile is not to be considered desirable, in fact, it is considered to be an indicator of inferior product quality. With substantial amounts of black coring, it is recognized as a potential source of: (a) mechanical weakness in clay products, (b) bloating, warping, or other unwanted deformations of the ware during firing, and (c) increased susceptibility to damage by weather conditions such as freezing and thawing.

Black coring in structural clay products is a phenomenon which has been known for many years: it dates back into history through the study of ancient ceramics. It results from the incomplete oxidation of the carbonaceous matter and transition metal ions such as iron and titanium. The materials responsible for the core formation are all natural impurities found in clay or shale. They include for the most part, carbonaceous matter as mentioned above, sulfides such as those of iron, and ferrous compounds. For example, pyrite (FeS2) is a common impurity in clay products. In the presence of little or no oxygen, it dissociates into FeS and sulfur at about 482 ºC.

The materials, firing conditions and reactions involved in the formation of, or elimination of, black cores have been studied by many authors. Basically, the process of core formation and subsequent elimination is governed by the availability of oxygen to the interior of clay wares during firing. The oxidation period. during heat-up of the ware, extends from 350 to 950 ºC (600 to 1740 ºF). At the lower end it overlaps the decomposition period of the clay minerals which can reduce the oxygen supply; on the upper end it may extend into the vitrification range which can also inhibit the oxygen supply and release the volatiles.

During this period of oxidation, the carbonaceous impurities such as coal, lignite, peat, etc., usually dissociate into volatile gases and carbon residue (char). If sufficient permeability of the ware allows oxygen to be present, there must also be a sufficiently high temperature, i.e. 550 to 750 ºC, to ignite the volatiles if complete combustion is to be realized. It is believed that with even a trace of carbon residue present in the ware, there is a potential for black coring and possible bloating. The removal of carbon is a time-consuming process, so it is helpful to oxidize at the highest possible temperature to take advantage of the accelerated reaction rates. After the clay de-hydroxylates it is likely to have the most open structure of the whole firing cycle, so the oxidation temperature should be above the decomposition temperature, but below vitrification or consolidation temperatures where gas penetration paths may be sealed off. The range of optimum oxidation reaction temperatures is then from about 870 to 980 ºC (1600 to 1800 ºF) for the various types of clay products. Often, a halt or slow rise in the temperature program, in this range, is needed to allow oxidation to take place even at the optimum temperature.

Accompanying the lack of oxidation in the core are often defects such as cracks or voids indicating a lack of complete consolidation or vitrification of the body. Black coring therefore, is considered to be an indicator of inferior product quality.