Step 1: Natural rubber is soft, sticky and less durable in its raw form.
Step 2: To improve the properties of natural rubber, it is heated with sulphur.
Step 3: This process is called vulcanization.
Step 4: During vulcanization, sulphur forms cross-links between rubber chains.
Step 5: These sulphur cross-links make rubber stronger, more elastic and more resistant to heat and wear.
\[
\boxed{\text{Sulphur}}
\]