Jolinda, as an instructor, as you say, should know that when using a casting mold you must fire to a higher temperature that just a slump. I have used these types of molds in the past and never have had a problem. A little bit of Boron Nitride and the piece falls right out.
As far as Delphi goes, I am fortunate enough to live in the vicinity and love going there in person. The products are just as they are described in their catalogs and on line. Their staff is more than willing to help if you have any questions or problems and they have always worked with me if I have been unhappy, which has been very rarely. I have never found a problem with their phone support either; if the person I am talking to doesn’t know the answer, they find out or direct me to someone who does know, and I have asked some very technical questions. I think that you need to give them a second chance, after all I do not believe that they were to blame for your error.