This could be the glass pane for example? But I'm looking specifically for the time period when glass was first used in art frames to protect the piece.
The use of glass in glazing to protect paintings would appear to date back to the mid 18th century:
...such was the interest in art from the mid eighteenth century, as expressed through attendance at exhibitions and academies, that the display of vulnerable old master and contemporary drawings, prints, pastels and watercolours became popular, by removing these works from portfolios, mounting and framing and glazing them.
Source: Stephen Hackney, The Evolution of a Conservation Framing Policy at Tate
This became necessary for display paintings due to the increase in air pollution:
Gradual industrialisation from the sixteenth century, accelerating during the eighteenth and nineteenth centuries, created air pollution throughout all urban areas...Most buildings were well ventilated by modern standards...and exterior pollution readily found its way in. Clothes, furnishings, fabrics, wallpapers and any absorbent decorative material would need regular dusting and washing.
Several techniques of art protection with glass predate the use of panes mounted in front of canvas.
The Greeks protected both sides of delicate gold leaf designs with glass:
Later, Orthodox Christians in Romania painted durable Christian icons on the reverse of glass panes, which can be hung in frames, protecting the artistic design: