temper water is added and mulled into the sand mixtures sodium bentonite sand mixtures are said to be u201c tougher u201d and not as u201c brittle u201d as calcium bentonite bonded molding sands prepared in the same manner it is also known to treat calcium bentonite with a sodium carbonate treatment a process known as peptizing to convert the calcium bentonite to a swelling sodium bentonite generally the clay or clay mixture is used in the sand e g silica sand generally about 3 to about 10 by weight based on the dry weight of the total sand content it is understood in the foundry industry that by adding more clay binder to a foundry sand mixture more water is also required therefore it is often the case that by using less clay binder in a foundry sand mixture and reducing the amount of temper water added the foundry sand mixture is just as strong as it would be with a higher percentage of clay binder and water in one embodiment the clay has a moisture content in the range of about 20 by weight to about 50 by weight preferably about 25 by weight to about 40 by weight more preferably about 30 by weight to about 35 by weight based on the weight of the clay after mixing the acid with the clay and sand in accordance with an important feature of the present invention it has been found that unexpected improvements in the foundry sand are achieved by acid treating one or more calcium bentonite or calcium.