To remove heat marks from slate, you can try the following methods:

1. Use a damp cloth: Wet a cloth with warm water and lay it on the heat mark. Leave it there until the cloth is no longer warm. Remove the cloth and dry the area.

2. Baking Soda Paste: Make a paste with baking soda and water. Apply the paste to the heat mark and let it sit for 15-20 minutes. Wipe the paste off with a damp cloth and dry the area.

3. Use a Pumice Stone: Wet the affected area and rub gently with a pumice stone. Be careful not to scratch the surface. Rinse the area with water and dry it.

4. Lemon Juice: Squeeze a lemon and apply the juice to the heat mark. Allow it to sit for 10-15 minutes, and then wipe it off with a damp cloth. Rinse the area with water and dry it.