Boiling your potatoes with salt or vinegar before you roast them can help make your roasted potatoes come out crispier, but there’s an even better way. All it takes is a little bicarbonate of soda.
According to J. Kenji López-Alt at Serious Eats, you should use Yukon gold potatoes, not russets or reds. They’re the perfect firmness for roasting. In Australia, Bintje potatoes are a good substitute. Slice them up into chunks, then get ready to rough them up with some boiling, alkaline water. Add half a teaspoon of bicarbonate of soda to about 2L of water and bring it to a boil. Add the potatoes, then reduce it to a simmer and cook for about five minutes.
When they’re done, the potatoes will be tender and the alkaline water will have broken down the pectin of the exterior of the potatoes so they’re covered in a starchy, potato slurry. That slurry will crisp up nicely when you roast the potatoes in the oven. You can learn more about the process, and watch a video recipe if you don’t have your own, at the link below.
The Food Lab: How to Roast the Best Potatoes of Your Life [Serious Eats]