I need ash to make healing salves. What is the best item to burn to get ash?

2 Answers

It's always interesting to burn things that:

• Have no (great) other use (@trollingchar's example of water pool plants is excellent)
• Are renewable
• Are easy to farm
• Stack nicely

For these reasons, I most often burn twigs:

• you can shovel saplings easily and replant them,
• you can plant them close together and farm them fast by holdingSPACE,
• they don't need manure to grow,
• and once you have 40 saplings planted, twigs come in aplenty.

Burning a stack of 40 twigs gives 40 ash.

Many items will do:

Ashes are an Item that is dropped after a flammable object has been burnt by Fire (some exceptions such as Trees), including the remnants of a Campfire and a destroyed Pig Torch or Fire Pit.
Ashes will also replace any item that is not allowed to be carried through to the next World via the Teleportato (e.g. the Eye Bone or Webber's Skull).

And, more appropriately:

Due to their abundance and minimal use, Pine Cones and Jungle Tree Seeds are good candidates to burn and get Ashes.

As well as tree stumps:

Stumps that have been burned will turn into Ashes.

3