In the process of investment casting, either silica or alumina based mixes are used as injected ceramic cores to provide complex cooling passages in the casting. For low temperature DS (direction solidified) applications, Si02 or silica is used as core material. In addition to good thermal shock resistance, silica has sufficient strength for the purpose and it can be easily removed by chemical leaching. In the case of high temperature alloys for SX (single crystal) applications, silica is not good enough and hence Alumina (Al203) based mixes are used, but they are extremely difficult to leach and hence Potassium Flouride based solutions are used, which are exremely toxic. Why are alumina based mixes difficult to leach ?