She's pretty sure spellsilver doesn't explode in water but it's not as if she's ever been in possession of spellsilver to handle destructively so it's not as if she's checked. 

She'll try to point to 57 when the proprietor is looking at Almys.

"I presume if it were something as obvious as quicklime-metal you would have already deduced the fact.  If we go rarer and higher and slightly less reactive and restrict ourselves to legal outputs, then I believe that could match 63, glowember?  Or perhaps 70, farglint?  I think one of those was the most reactive from that long stretch of silvery, easily bent metals from 57 to 71, but I don't recall which."

The layout of the shelf does not quite make it obvious whether boyCarissa pointed to 45 or 57.  "Any chance it's rosegleam?"

"That's one of the least corrodable metals.  They're putting it in half the modern alloys now that it's cheap, lets armor shrug off Acid like rain.  The weaker Acid attacks, anyways.  Think you probably got something mixed up, there."

"Yeah, sorry, brain-dodgefail, not rosegleam.  Huh.  If we were just considering that stretch of silvery metals in order, what exactly is 57 hiddenglint like?"

The proprietor makes a face at this, but she obligingly digs out a book from under her store-counter, the sort that looks just thick enough to be 92 pages long.

Yes that one. Also how can you have 92 pages to say about it while not noticing what it is??

The proprietor flips to the 57th page of this handbook of all the elements!  Carissa might be overestimating how much attention this world gives to spellsilver in particular.

"Hiddenglint.  Silvery, fairly soft, malleable and ductile.  Tarnishes rapidly in air within hours, forming a white crust that flakes to expose fresh metal that's then consumed, don't make anything permanent out of the pure metal.  Powder readily ignites.  Melts at 1190 morrisons, forgeable.  Not significantly toxic.  Not magnetic.  Not part of any known alloys on the frontier of useful properties."

"That could... maybe be it..."

The proprietor taps the adjacent page.  "The entry for sparkstone is nearly the same except that it's slightly magnetic, forms a more yellowish tarnish on air exposure, sparks when you strike it, and if you compress the metal hard enough it'll collapse in volume by around a sixth."

Almys sighs, trying to make it sound as dadlike as he can manage.  "Your memory is better than mine sometimes, Kellum.  Did you happen to look over while Daddy was working and remember exactly whether the tarnish looked a little yellow or not?"

"It wasn't yellow. But you should get the yellow one! A yellow one would be fun!" Hopefully they are sufficiently on the same wavelength despite the culture difference that he will understand he is supposed to slap a child for that, not listen to them.

Why is his life like this... his liege probably doesn't mean for him to be visibly following a child's orders, right.  Right?

He's not really sure he is picking up all intended subtext.  ...Maybe she found a different element she wants?

"I don't suppose you've got a copy of that you can loan with a return-deposit for an hour so I can look through it and think?  Or two copies, it'll keep Kellum busy.  He's a big reader for his age, has never hurt a book."

"Got a guild card I can have a look at?"

He does not particularly want to leave a trace saying 'Almys was here', especially after introducing himself with a different name... his liege can not-Illusion it, maybe.

"I'd have to come back with it, but I could.  Not a huge fan of the inconvenience.  Return deposits not enough?"

"Not enough to pay for my inconvenience if I've got to replace it."

"I'll trust you with an excess deposit, since I'm planning to do a bigger deal than that with you."

"I've only got the one book.  Don't hand it over to the kid, and sure, you can have the book for an hour.  Slow day anyways.  2-round deposit."

"I give you my word I shall not let any child touch it."

And soon enough the dad has led his little boy out of the Transmuter office, down a street, and into an alleyway where there isn't anyone obvious who'd be watching them or listening to them.

"57. I think also some of the ones that follow it, but definitely 57. We can't get it that pure."

"Ah.  I thought -- well, first I thought maybe you wanted me to get some of a yellow element too, like gold.  And then I thought maybe you were saying something deliberately confusing, because you wanted me to get you out of the store before your current disguise ran out, or because you'd realized you wanted to look through that booklet yourself."

"Also it's just now occurring to me that maybe the mundane metals in a blade interact with enchanting it.  And that before Transmuter Devices, daggers were made of iron or steel, instead of rhinemetal-adamantine blades with godbone-starmetal edges.  Should we get iron, too, for your recipes?"

"You do some things differently to put spellsilver in mithril but not importantly differently. If it's inexpensive we can get some iron but we should primarily get whatever you'd expect a dagger to be made out of these days, I expect I can work with it. ...well, I'm not a smith, so I expect I can work with it if we can hire a smith or if there's local magic for it, and otherwise we should just buy an unenchanted dagger rather than any pure metal at all."

"And now I realize how little I really know about modern smithing.  I know that the good alloys have weirder rules than 'melt all the component metals together, and pour them into a dagger-shaped mold around a core of affinity mats under a temporary Ice spell'."

"Do you remember which elements you were confident you could work with?  Is one likely to be more powerful or useful than the others?  I'm wondering if we should look for one that the handbook says rusts less rapidly in air.  Storing something volatile probably costs extra... I should've asked after the price of that."

"We just keep it in oil, and the oil's not an important part of the expense. But I think all of these -" she flips through the book all the way to lastglint - "should work."