You can’t use someone else’s sigl.
More than any of the other four, the Blood Limit shapes the drucraft economy. It is the primary reason that sigls are so expensive.
Just like the gemstones they resemble, sigls can last a very long time. They’re short-lived in geological terms – aurum naturally sublimates into free essentia, and if left for long enough, a sigl or a lump of aurum will wisp away into nothingness. However, ‘long enough’ on a human timescale is a very long time indeed, and a solid sigl can easily last for thousands of years.
This raises an obvious question – why isn’t the world filled with sigls? Nowadays the vast majority of sigls are threaded rather than solid, with exponentially shorter lifespans, but even if only five percent of the ones made each year were solid ones, that’s still an enormous number that would only continue to accumulate century after century. So why are they in such short supply?
The simple answer is that a sigl made for one person won’t generally work for another. There are indeed vast numbers of sigls in the world, sitting in museums and bank vaults and private collections – it’s just that the vast majority of them were made for people who are now long dead, and are therefore completely useless to everyone else. Thus, while the absolute lifespan of a sigl can be measured in thousands of years, its practical lifespan is, in generational terms, very short.
Anatomy of a Sigl
Sigls are comprised of three layers. The outer layer, known as the shell, is an ablative and largely inert coating designed to protect the sigl from erosion and damage. The middle layer is referred to the body or outer core, and comprises the majority of the sigl’s mass. However, it is the inner core, otherwise known as the kernel, that is the reason that the Blood Limit works the way it does, because a small but critical fraction of the kernel is made up of the wielder’s personal essentia.
Shaping a sigl with someone’s personal essentia is something like forging a lock with a human key. It’s the only reason that sigls work at all. Making a sigl without any personal essentia all just produces an inert lump of aurum, and trying to use a sigl whose personal essentia doesn’t match yours gives the same result.
This means that if you want a sigl, you can’t buy one second hand. You have to either make one yourself, using your own personal essentia to shape the kernel, or have a professional shaper take a sample of your personal essentia to make one for you. Most people pick the second option.
The result of this is that the drucraft economy doesn’t work like the markets for manufactured goods such as jewellery or furniture, where the products can (in theory) be maintained indefinitely. Instead, it follows a cycle where new sigls are created, remain in circulation for a finite time, then disappear.
Workarounds to the Blood Limit
The Blood Limit is probably the most extensively studied of all of the Five Limits, and the workarounds to it are well-understood.
The simplest workaround is to have the shaper, when they create the sigl’s kernel, mix in someone else’s personal essentia with their own. Creating such ‘mixed sigls’ is inherently more complex than creating a pure one, but the techniques for doing so have been exhaustively practised over the centuries and the procedure is now quite routine. However, you don’t get something for nothing: the more of someone else’s personal essentia there is in a kernel, the weaker the sigl will be when you try to use it yourself. This method is thus less of a true workaround and more of a compromise.
The second (and more famous) method is to use a sigl belonging to someone whose personal essentia is sufficiently similar to yours. This generally requires the sigl’s creator to be a blood relative, the closer the better. In this way sigls can be passed down from parent to child, or from sibling to sibling.
This method is not infallible. The closer a blood relation two people share, the more similar their personal essentia tends to be, but there’s still variation. Parent to child or brother to sister almost always works, but it’s not a guarantee, and as the relationship becomes more distant, the chance of the sigl working drops like a rock – two ‘steps’ in the family tree is usually the realistic maximum. And this assumes a pure sigl, rather than a mixed one. If the sigl was shaped by someone else, the personal essentia in the sigl is effectively diluted twice over, reducing the viability even further.
Still, even with its drawbacks, the ability to pass sigls down to the next generation is incredibly powerful. Even if you can only get two generations worth of use out of a sigl before it becomes useless, that still represents an enormous advantage over anyone who uses a sigl for only one. The concept of inherited wealth takes on a whole new meaning when children can inherit not only a house and money from their parents, but magical powers as well. This is a major reason for why the equivalents of Drucraft Houses developed independently in so many places around the world . . . and even in countries with no House tradition (or in ones where such practices are specifically banned) family-run corporations and political and financial dynasties fill the same role, passing down their wealth in exactly the same way.