A modern alchemy claim, checked
Can fusion turn mercury into gold?
Marathon Fusion is a fusion energy startup. In July 2025, it published a paper. The paper says fusion plants could turn mercury into gold. A fusion plant's main job is electricity. The gold would only be a by-product. This page checks that claim.
Scientists first turned mercury into gold in 1941. The reaction is well known.
The numbers come from computer simulations. Other scientists had not yet checked the paper.
The plan needs a fusion power plant. No such plant runs today.
First, the big picture
A fusion plant makes electricity. Gold is a bonus.
Marathon Fusion does not build fusion reactors. It designs fuel and blanket systems for future fusion plants. Its gold idea fits inside those plants. Follow the loop below.
Simplified. Real blankets mix these layers, and some designs use beryllium or lead instead of mercury.
Fusion
Deuterium and tritium fuse in a hot plasma. Each fusion releases energy and one fast neutron.
Heat becomes electricity
The neutrons hit the blanket around the plasma. The blanket gets hot. The heat makes steam. The steam spins a turbine. This is the main goal.
Mercury multiplies neutrons
One neutron goes in. Two neutrons come out. A little gold forms here.
Lithium makes fuel
A neutron hits lithium. The lithium splits into helium and tritium.
Fuel goes back in
The tritium is collected. It returns to the plasma as fuel. The loop starts again.
From star fire to your light bulb
How fusion makes electricity
Fusion does not make electricity directly. Fusion makes heat. The heat boils water. Steam spins a turbine. The turbine makes electricity.
Simplified and not to scale. Some designs skip the water and use a gas turbine, but the idea is the same: heat → spin → electricity.
Fusion
Hydrogen atoms fuse inside a ring of hot plasma. Each fusion releases energy. Most of it leaves with a fast neutron.
Heat moves
Neutrons crash into the blanket. The blanket gets very hot. A coolant carries the heat away.
Boiler
The hot coolant passes through water. The water boils. It becomes high-pressure steam.
Turbine
The steam pushes on turbine blades. The turbine spins very fast.
Generator
The turbine spins magnets inside copper coils. Moving magnets push electrons along the wire. That flow is electricity.
Your home
Power lines carry the electricity away. It reaches homes, schools and phone chargers.
Where the energy comes from
The helium and neutron weigh slightly less than the fuel did. About 0.4% of the mass disappears. That lost mass becomes energy.
The neutron carries most of it
The neutron takes about 80% of each fusion's energy. The blanket catches it as heat. The helium keeps the plasma hot.
Tiny fuel, huge energy
One gram of fusion fuel holds about as much energy as 8 tonnes of oil.
Coal, gas and nuclear plants all work this way. They all boil water and spin a turbine. Only the heat source is different. Fusion uses the same fire as the Sun.
Why mercury?
Mercury and gold are next-door neighbors
The number of protons decides the element. Gold has 79 protons. Mercury has 80 protons. So mercury is one proton away from gold. Lead is three protons away.
Heat and mixing only move electrons. They never change the nucleus. Only a nuclear reaction can do that.
The recipe, step by step
From mercury-198 to gold-197
The clever part
Why gold could be a free bonus
A fusion plant must make its own fuel. It makes this fuel with neutrons. So every neutron is valuable.
One neutron in, two neutrons out
A fast neutron hits mercury-198. The hit knocks out a second neutron. One neutron goes in. Two neutrons come out. So the plant gains a neutron.
The fuel and the gold, together
Fusion plants plan a layer that multiplies neutrons. This layer is usually lead or beryllium. Marathon Fusion wants to use mercury instead. Then the plant makes fuel and gold together.
How much?
Their estimate, against the world's gold
The company made an estimate. One large fusion plant could make about 5 tonnes of gold a year. That gold is worth hundreds of millions of dollars. Still, the world's mines produce far more.
Both bars use the same scale. One plant would add about 0.14% to the world's yearly gold. It would not crash the gold price. It could help pay for fusion power.
The catches
Why there's no fusion gold yet
Fusion plants don't exist yet
No fusion plant sends power to the grid today. The gold plan needs a working fusion plant. So the gold must wait for fusion.
It's a simulation
The numbers come from computer models. The paper is a preprint. Other scientists had not checked it yet. Nobody has built the full setup.
The gold would glow, a little
Neutrons also make unstable atoms. Some of the new gold is radioactive. The gold must sit in storage for 7 to 17 years. Then it is safe to sell.
Only 1 in 10 mercury atoms works
Natural mercury is a mix of isotopes. Only about 10% is mercury-198. Only mercury-198 makes stable gold. The other isotopes make unwanted by-products.
Mercury is toxic
Mercury is poisonous. The plant would hold tonnes of it. The mercury would also become radioactive. That makes it hard and risky to handle.
History
Alchemy that actually worked
The alchemists try chemistry
Alchemists melted and mixed mercury, lead and sulfur. These methods never reached the nucleus. So they never made gold.
Mercury becomes gold in a lab
Harvard physicists fired fast neutrons at mercury. They made a tiny amount of radioactive gold. This was real transmutation.
Bismuth becomes gold
Glenn Seaborg's team turned bismuth into gold. They made only a few thousand atoms. The gold cost far more than it was worth.
Lead becomes gold at CERN
Lead nuclei flew past each other at nearly light speed. Some lost three protons. They became gold for a split second.
Gold as a fusion by-product?
Marathon Fusion proposes gold by the tonne. First, fusion plants must be built.
The lesson
Chemistry rearranges bricks. Nuclear physics changes them.
Mixing only moves electrons. So mixing can never make gold. A fast neutron can reach the nucleus. Fusion makes such neutrons. The Sun runs on fusion too. The alchemists' dream is real physics. It needs a star's furnace, not a stove.