mixture using the preferred lithium carbonate the clay mixture will have a ph of a least 7 most usually in the range of 8 to 10 the lithium exchanged clay material is discharged from the pugger usually dried and then heated at a temperature high enough to collapse irreversibly the mineral structure by as low as possible to achieve this result when temperature is too low e g 125 u00b0 c the structure cannot be collapsed using reasonable durations of heating on the other hand when temperature is too high e g appreciably above 300 u00b0 c the resulting pigment has undesirable coating properties and initial sensitivity and image fade resistance are adversely affected also pigment brightness is undesirably decreased a lower limit of temperature appears to be in the area of 160 u00b0 c when the duration of heating after drying at room temperature is 24 hours an upper temperature is about 300 u00b0 c recommended are temperatures in the range of about 200 u00b0 c to about 250 u00b0 c for about 30 minutes to 24 hours especially preferred are temperatures in the range of about 220 u00b0 c to 250 u00b0 c for 1 to 4 hours after drying reduced charge montmorillonite as synthesized is in the form of chunk like aggregates it is essential to subject these aggregates to ultrafine grinding with optional classification before using the reduced charge.