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Vasart is the continuation of Ysart glass by the same family that made Monart at Moncrieff.
Vasart is mostly produced in different colours and shapes to Monart and that is the main difference. From 1946-56 Vasart was signed but from 1956 to 64 they used a label. Vasart then Became Strathearn Glass at a new location and the last of the Ysart's left Strathearn. The Vasart shapes were continued at Strathearn but because of technical changes and colour supplier that is usually distinguishable from Vasart.
Just to confuse the issue there were a few contemporary makers and some descendants that produce glass in a similar style the so called "Ysart Style".
Values are variable because Monart was made in very small quantities as a part-time activity of the Ysart family at Moncrieff. Passing time is revealling some remarkable Vasart which competes on price with Monart, though generally Monart is the more expensive, particularly the very early production and lighting. Vasart is starting to increase in value as the scarcity of Monart makes Vasart and Strathearn more affordable alternatives in the style of glass.
However, compared to other Art Deco glass (Lalique, Schneider etc) Monart is relatively cheap - and prices are not highly consistent. This is because Monart was at the time it was made an expensive glass - each piece was hand-blown and shaped and there is not enough in existence to support a large enough community of collectors that would stabilise prices. However, the combined range of Ysart Style glass is sufficient to create a market and more and more collectors are always appearing. The market for Ysart glass in the USA has started to grow and in coming years this could lead to prices that make today's look like bargains. There will be little difference in price between the best Vasart and the 'Medium' Monart, leaving a reasonable pool of less exciting pieces affordable.
There are other types of glass not wildly dissimilar such as Nazeing, Gray-Stan and some lesser British glassworks that also are growing in popularity.
The ysartglass.com website provides sufficient information to help collectors identify the differences between the various types/periods in most cases. Although it is a lot of data there, a short guided tour is there for first time visitors.