Patenting an added ingredient as an improvement

Question

Someone has an expired patent on a mixture that makes an oil painting medium. I will not use real names, but here are the ingredients of it

Painting oil  
+ Stone dust (gives medium a certain quality to it).

I want to make that patented medium thicker, by adding one ingredient, a third mineral dust.

So I would like to get patented this:

Painting oil  
+ Stone dust  
+ my mineral dust thickener 

If my medium behaves a little thicker than the previous patent's medium, is the difference enough to enable me to patent my mixture and then sell it as a product in the market or license the patent to producers?

Answer 1

Yes, you could apply for a patent for the new combination. However, it might be refused as being obvious.

The expired patent (P1) which discloses the combination of painting oil and stone dust is prior art. This is true whether P1 is expired or not. Because of this, your invention would need to be non-obvious over P1. Since the only difference is the addition of mineral dust thickener, the key question would be: is it obvious to add the mineral dust thickener to the product of P1.

If there is another document (P2) which suggests that the mineral dust thickener is useful for thickening paints, it would probably be obvious to combine P1 with P2 to arrive at your invention. In other words, your combination would be obvious, and therefore not patentable.

On the other hand, if the mineral dust thickener has never been known to thicken oil-based products (or something like that), the argument could be made that adding it to the combination of P1 is a non-obvious thing to do. Because of this, your product would not be obvious, and so would be patentable.