Gray’s Formulary contains a reliable cleaning procedure for slides:

40 parts saturated aqueous potassium dichromate

Add, with due precautions, 60 parts sulfuric acid.

I always just put excess dichromate in 60 parts water so that much solid dichromate remained in the bottom of a coverable

 Container.

Add the 60 parts of Sulfuric Acid, with care.

The quantity of cleaner may be a gallon or so, mixed in a container kept for that purpose.

Put the coplin jars or other glass ware in the mixture for a few minutes or over night.  Rinse the glass well with deionized water.

This mixture can be used for a long time.  But the best procedure is when the solid dichromate is gone, a fresh batch is mixed.