This is slurry is made of 80% Redart and 20% KT1-4 ball clay. It has very good casting properties IF it is deflocculated well (smaller pieces can be cast in 10 minutes and extracted from the mold in another 10). However high-iron slurries are notorious for gelling, either right away during mixing or later. Sometimes a slurry seems to pour well into a mold but 10 minutes later it has gelled so much that vigorous shaking is need to loosen it enough to pour it back out (even then much may hang on in corners). A batch of slurry may need extra deflocculant several times over the first few days of being used. Davan 811 is often used for these. More deflocculant is needed, in this case about 1 gram for each 100 grams of powdered clay.
The deflocculation process is the magic behind the ceramic casting process. It enables you to make a slurry of far lower water content and thus lower shrinkage.
The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.