The backsprings on GEC Tidioutes & Northfields are carbon and take on a very nice even patina. I cut two oranges for juicing at breakfast every morning and I use a GEC knife that I want to patina. After juicing the oranges I cut up the skins so that they compost better. This method gets a lot of juice on the hands and backspring too. Pears are particularly good for getting patina, peaches and nectarines as well. Worth noting that even a small knife such as a Conductor 33 has a sharp and credible blade that will deal with a large orange with ease. Backsprings of my Tidioute Dogleg, Conductor, 73 knives and Queen QCCC Teardrop have all taken very good patina in this way. Hope this inspires you!