NB: These guys are THE best in the business; their service, selection, and the interactive tools on their site are second to none. Check their blog for loads of info, how-to videos and more. (No affiliation, just a loyal and satisfied customer!)
NB: These craftsmen make magnificent glass dip pens. They are pricey, but flawless in their crafting and performance. Truly an heirloom quality glass pen. Well worth the price. Their service is also top notch, including repair of my pen when it was snapped by a user unfamiliar with glass pens. (No affiliation, but a happy customer.)
