My Verditius glassblower character wants to meld gold (or other valuable metals and/or gems) in glass so she can increase the amount of Vis she can use to open the item for enchantements. She would then be able to use the material and size of the precious metal instead of those of the glass object. IE : make a glass shield that had about a ring of gold incorporated in it so it would allow 110 vis (very small gold object) instead of just 41 (large glass object). That's because she has a major magical focus with glass and would like to make major enchantments made out of glass.
Now the question : Would it be a Muto Terram with Range Touch, Duration Mom and target individual to incorporate the gold in the glass definitively (you would need both objects), or a Mystery/Secret glassblower knowledge that would rather use a Rego Terram Range Touch, Duration Concentration, Target individual ? And what would be the base level ?
Or would the Target have to be part or group (because it fuses both the gold and glass) ?
I think it's a great idea and I would really like to help my player design this and at the same time I want to keep the system coherent and as close to the rules as possible (I'm not against some rule bending now and there, just want to avoid it if possible).
Red glass of the period was made by infusing gold, however the amounts involved are miniscule, so I would say that as long as the substance is still glass it uses the glass guidelines. If there is enough gold for it to count as a precious metal vis receptacle either the piece of glass is huge or it really isn't glass anymore but some weird gold-silicone alloy...
though at the 13th century copper was used far more commonly to color red glass than gold...
though an interesting possible outcome of this experiment- "ruby gold" glass was for a time considered a valuable substance o its own, and it would not be impossible in ars magica for it to be treated as a precious gem for the purposes of storing vis- a sort of thaumaturgical synthetic ruby.
momentary rituals don't work for uto. What you would need is rego craft magic to meld them, possibly involving muto stages-
for example as an odd thought experiment, consider what would happen if you converted an iron sword to wood with MuTe(He) and then burned it. When the spell runs out you would have a fine pile of rust (oxidized wood). Similarly if you conver glass to water and gold to salt and mix it to a brine then use ReTe to mold the resulting material when you are done...
As far as fusing them into a single item, you get some tricky options. An easy possibility for your glass shield is to have a simple web/ring/rod of gold inside the glass, and use that as your opening (just like opening the gem at the end of a wand can enchant the entire wand as an item). You can also utilize an often-forgotten aspect of Verditius magic:
I'm already looking forwards to having my Verditius experiment with metallic Muto options, and earlier in the forums I had a post about MuMe(Te) to turn someone's mind into a weapon.
Guys, as far as fusing them as desired, it's trivial. Look at the core rules for Verditius Magic. It says right there Verditius Magic allows you to craft things in ways that aren't normally craftable at all. If this weren't a Verditius, things would be trickier.
Edit: Here is the rule with the example, which happens to be very similar to this, a gem inside wood instead of a golden ring inside glass.
Callen and racoonmask already provided the answer that is provided in the core book. So, yes, it is doable, it does not require any special virtue or skill as long as you are Verditius.
If you want to go one step futher, you could consider that as long as it is easy to identify the various components of a magical item (easy = doable by a trained magus without requiring specific spells and/or investigation), then it can be done as described by the quoted rule.
However, you can consider that for a mage who would like to be more discret or tricky, he might be able to infuse the magical capacity from an item into another one, but without being noticeable without a laboratory investigation. This might requires a special initiation, a minor research or a special virtue.
In the case of the glass/gold thing mentionned by the OP:
Option 1: the glass is clearly doted with tiny gold crumb, or the item has a golden rim.
Option 2: through this mysterious ritual/operation, the glass acquires the vis capacity of gold, but it just looked like mundane glass. The gold has been consumed during the infusion process, and the quantity used determine the new vis capacity of the item, overriding its previous capacity. It does not get any other properties. Skilled mundane alchemist might find this substance strange but may or may not be able to infer what was done to it or how to use it (if it ever has special capability).