Cranfield (Spectrum) Spectragel Medium

From the Cranfield website:

This is a thixotropic alkyd medium designed to mix with oil colours, retaining the body of the paint. Spectragel is a gloss medium and is quick drying. It is a clear medium based on a synthetic resin that improves the weather resistance of oil colours; ideal for glazing and extending colours while retaining gloss and structure of paint.