For what it is worth....
... dates can be a very difficult to get accurate and can only be confirmed from verifiable first hand, witnessed accounts. Even dealing with the families and workers related to a single glassworks can be fraught with inconsistencies.
On my site I have three highly researched resources - listing company data, people data and event data. Even 20th century official records such as birth, wedding and death certificates can include conflicting dates, parentage and even spelling. Company dates can be an even bigger nightmare.
The 19th century material is even worse.
The worst source is word of mouth, particularly from the people themselves. If they kept a daily diary then that might be accurate but recollections lead to all sorts of problems. Because of the number of dates that have only had a single corroboration, there exist many question marks against dates in the aforementioned resources... These question marks are against dates that have some documentary evidence. Quite often is up to the author of a work to pick which 'documented' date to use. I have many cases where documented dates can vary by up to 10 years and those selected are chosen in context with other dates and with reference to the date that date was written down. So birth certificates are more readily accepted for date of birth than wedding or death certificates - but a family bible entry could be regarded as even more reliable. Worst source is a workers CV where dates tend to be adjusted to fill gaps and/or to enhance a particular experience.
Surely for the interest of collecting, what counts is the period that the people worked and produced various pieces - this carries a useful message about whether the subject was a radical progressive or a follower of fashion in their work.
Accurate dates become important when there is a need to define a 'first' event from two closely competing claims - a glance at any history of invention will show that these types of battles are very commonplace and inevitably one party must be lying. Yet each party may honestly believe they are right and the decision will rest on the 'most convincing' explanation/evidence.
A piece of glass on eBay claims to be a very early example of Monart Glass dating to c. 1880 when Salvador was just two years old. This creates a problem, as according to research he did not start working in a glassworks until he was 9 years old. That he had developed Monart 7 years earlier is a major surprise and has consequences for the whole art glass movement - GallÃ© and the Nancy school were it seems just copying the concepts of the young Salvador Ysart. :twisted: