A lot of people (like me) love to throw the term around but it’s seriously ill-defined, so let me explain to you what makes it so special that buying it new starts in the three figures a yard.
First of all lampas is somewhat new word. If you went into that hypothetical time machine then asking for lampas silk in the 18th century would net you perhaps something in one or two places but most people would be, like, “whut?”
As for nowadays, Wikipedia tries to tell us something about lampas but it’s not really informative:
Lampas is a type of luxury fabric with a background weft (a “ground weave”) typically in taffeta with supplementary wefts (the “pattern wefts”) laid on top and forming a design, sometimes also with a “brocading weft”. Lampas is typically woven in silk, and often has gold and silver thread enrichment.
Basically what they are getting at is this:
Together weft and warp make the basic fabric weave.
So how to we get from this basic weave to “lampas”?
Most lampas fabrics are multi-colored, so the weaving has to coordinate a lot of addtional weft threads. It’s also very materially intense, since you need way more silk for this fabric.
Now where is the difference to silk brocade? Well, brocade is super ill-defined. If you wanted to, you call all lampas brocade since the secondary weft going in and out the fabric is “brocading”. Not all brocade is lampas though, obviously. Lacking secondary and tertiary warps is a dead giveaway.
Unfortunately seeing and identifying those secondary and tertiary warps is really difficult. If they have the color and thickness of the base fabric, it becomes quite difficult actually. So the easiest way to spot a lampas is to be told that it is one by its seller.
I know that’s an absolute non-answer. But the likelihood of encountered new silk fabric that is lampas but not labelled as such is pretty much non-existent. So if something new is merely labelled “brocade”, it’s probably not lampas. It’s like labelling a Ferrari “a car” which is stupid from a marketing perspective.
On vintage fabric this obviously doesn’t apply because there are genuinely sellers of lampas fabric who have no idea what they have. The rule of thumb is that you should not buy it on the hope that it is the “right” weave. As previously mentioned on that blog merely something because it ticks the right boxes (like worsted pure wool) doesn’t mean you will like the fabric, let alone working with the fabric. Buy something because it looks cool and nice and beautiful and fitting your project.
Even then, this might net you a polyester brocade (they exist and from afar they can look really good) but at least then you’ll have something pretty.