A European treatment shop that applies a chemical conversion coating to an aluminium component for an aircraft engine, a landing gear actuator or a heat exchanger is operating inside two control systems at once, and the two do not speak to each other. The first is chemicals law, which decides whether the substance in the tank may lawfully be placed on the market and used at all. The second is the customer’s own quality architecture, which decides whether the treated part will be accepted for the programme it is destined for. Each can say no on its own. Neither can say yes on behalf of the other. For most of the industrial base this distinction is invisible, because the same processes have been running under the same approvals for decades. It becomes visible, and expensive, at exactly the moment European governments have chosen to expand output: when a substance has to be replaced, when a second site has to be brought on, when an incumbent supplier fails, or when a regulatory regime is dismantled and rebuilt underneath a supply chain that cannot stop. The question that follows is not whether hexavalent chromium can be replaced in surface treatment. It can, in defined applications, and it has been. The question is what separates an available substitute chemistry from approved productive capacity, how long that distance takes to cross, and who is exposed while it is being crossed. That question has a documentary answer, because both the European Commission and the British authority have had to give one in writing, under legal tests that force them to say precisely what they mean by an alternative being available.