An American chemist whose tips for making the perfect cup of tea include adding a pinch of salt has prompted a diplomatic intervention from the US Embassy.

Michelle Francl is a professor of chemistry at Bryn Mawr College, Pennsylvania, and a lover of English tea. In a research quest for brew-topia, she has written a book, Steeped: The Chemistry of Tea, published by the Royal Society of Chemistry.

Before arriving at her perfect cup conclusion, the celebrated scientist studied more than 500 research papers and undertook testing methods dating back to the eighth century, including trying to decaffeinate tea with vodka. While most tips are uncontroversial, such as do not heat water in a microwave and do not let the tea brew for too long, one in particular has raised eyebrows, causing the US Embassy in London to step in.

In a statement posted on X social media platform, the embassy said, "Tea is the elixir of camaraderie, a sacred bond that unites our nations.

"We cannot stand idly by as such an outrageous proposal threatens the very foundation of our special relationship. We want to ensure the good people of the UK that the unthinkable notion of adding salt to Britain's national drink is not official United States policy. And never will be."

On writing the book, Francl said, “Writing Steeped was great fun, a chance to combine my love for chemistry and my delight in a really good cup of tea. Even after all these years of drinking tea and doing research in chemistry, I learned new things about what is in my cup and how to make the very best cup of tea.

“I enjoyed the challenge of explaining it in a way that people who are not chemists can appreciate the beauty of the chemistry happening and can use what they've learned to make a great cup of tea.”

Speaking to the BBC about the salt furor, she said she "certainly did not mean to cause a diplomatic incident".

"My emails have been going crazy today. I did not anticipate waking up this morning to see loads of people talking about salt in their tea."

Dr Francl's scientific tips for making the perfect tea include:

Reduce the bitterness in your cuppa with a tiny pinch of regular table salt.

Warm milk is best. If you use a teabag, add milk after pouring the tea and, if possible, heat it gently before mixing.

Short and stout cups keeps your tea hotter

Dispose of used tea leaves and teabags after one use - virtually all the caffeine and antioxidants in them are gone within a minute of steeping.

Heat up the cup (or pot): Pre-warming a teapot helps it to maintain a higher temperature while steeping.

Never put tea in a microwave. Water boiled in the microwave reaches its boiling point so quickly that little of the dissolved oxygen or carbon dioxide has a chance to escape, resulting in a higher likelihood of a scum forming.

Longer steeping means bitter, not better: Caffeine is hydrophilic, which means water-loving, so it is released quickly but some of the bitter compounds – for example, tannins – do not dissolve as quickly. This means that if you leave tea to steep in very hot water, your caffeine hit won’t change much but the bitterness will.

Use a fabric or felt tea cosy on a teapot and try to cover your cuppa to keep it hotter for longer. The process of convection, where fast-moving molecules move away from a hot object and take some of its energy with them, will see it cool quicker otherwise.