Topic: Help needed please.... Black Glass question.

[Ohio]

Two US manufacturers made pure ebony/black, Pairpoint & Erickson & there has  been speculation that Carl Erickson may have brought the formula with him after serving his apprenticeship & 20 years with Pairpoint. Ken

[jsmeasell]

We make a great black color here at Fenton, and the key ingredients are manganese dioxide, cobalt oxide, and potassium dichromate. Incidentally, black glass is quite "soft" and takes longer to "set up" when our glassworkers are making pressed or blown items, so the required number of pieces to be made in a turn is LOWER than the numbers for some other colors.
 

[Paul S.]

thanks Anne  -  my days of lapidary and gemology are far behind me, but from memory I don't think there is such a beast as a 'black' agate anyway, although it wouldn't surprise me if they hadn't managed to dye one blackish at some time.    You often see massive book end lumps of half geodes stained blue or yellow, and showing the attractive interior crystals.   The quartz group are notorious for not 'taking' dyes (too dense and hard)  -  which is why they sometimes irradiate colourless quartz crystal clumps to give them a smoky hue, and palm them off on unsuspecting tourists as Smoky Quartz from the Cairngorms
Sorry Lustrousstone, I had forgotten to thank you for your help, my apologies.   If you were on hols., hope you had an enjoyable break.
Now, about this black glass...............

[Ivo]

When I was making jewellery many years back, the pure black stone was black agate and the ones with swirls, streaks and blobs in were defined by variant names such as banded agate or snowflake etc...  so I've always understood black agate to be a plain black stone...  but I do think it's one of those areas where different folks use different names and confusion reigns! Perhaps it should have been better described as obsidian like rather than agate like.  

Hush don't let my inhouse geologist hear it. Agate is defined by its structure and comes in many colours - solid or banded. Obsidian is volcanic glass. They are not related.

[Anne]

Yeah I know Ivo, and I was hoping you (or your inhouse geologist!) might comment...   It was more the colour than the structure that I was commenting on... pure black, you see.

[Adam]

Almost any colouring oxide if enough is used will give something which for practical purposes looks black.  The one used mostly (and by us, always) was manganese dioxide.  It is effective and cheap.  Black was also a good way of getting rid of any old cullet of any colour, including enamel decorated.  If not too much was used at once the manganese would easily swamp the diluting effect of lighter coloured cullet.

Adam D.