What are borosilicate glass jars?
For a glassblower who has already mastered the fundamentals of pipe making, jars represent a natural and exciting expansion. The same heat management and rotation discipline you learned making spoons and Sherlocks applies directly — but jars demand more of it. You're working with greater mass, taller forms, and a sealed base, all of which require a more patient and deliberate relationship with your torch. The reward is a product that sits at the intersection of craft and utility: something people want to own, display, and use daily.
As a commercial form, artisan borosilicate jars command strong prices. A well-made heady jar with fume or color work routinely sells for $80–$200 at festivals and through platforms like GlassPass. The form also lends itself to customization — different body shapes, base profiles, surface treatments, and lid styles give each piece a distinct character. Mastering the jar opens a whole range of vessel-making skills that carry into goblets, vases, and other decorative forms.
Watch the full jar-making series
Glass Jars Vol. 1 and Vol. 2 cover the complete process — base construction through advanced vessel forms — with a master artist walking every step.
What glass and tools do you need to make glass jars?
The tubing size is the first decision. Most small to medium jars start with 38mm or 44mm tubing — enough interior volume to create a useful vessel without becoming unwieldy to work at the torch. For larger jars or wide-mouth forms, 50mm tubing is common. See our glass sizes and buying guide for a full breakdown of tubing diameters and wall specifications — wall thickness matters more for jars than it does for pipes, since thicker walls produce sturdier finished pieces and anneal more evenly.
Your torch needs enough surface-mix power to heat large-diameter tubing to a workable state without rushing the piece. A mid-range oxygen-propane torch like a GTT Bobcat or Nortel Minor Plus handles most jar work; larger jars may benefit from a more powerful head. The equipment guide covers torch selection in depth. Kiln chamber size is often overlooked: check that your kiln has enough internal height and width to accept a finished jar standing upright before you start the project. Our kiln buying guide lists chamber dimensions for popular models.
How do you form the base of a glass jar?
Start by choosing which end of your tube will become the base. Cap one end with a plug of clear rod or use your graphite paddle to push the molten end closed. Heat the bottom inch of the tube slowly and evenly, rotating constantly, until you see the glass sagging slightly inward and the end beginning to close. At that point, press it gently but firmly onto the flat graphite marver — the surface should make clean, even contact across the whole base diameter.
The seal is the critical step. Bring the piece back to the flame and heat the edge of the base in short rotational passes, working inward from the perimeter toward the center, until no seam line is visible when you hold the piece up to the light. Any visible seam or thin spot in the base is a potential crack initiation site — it will survive the torch but may fail under thermal stress later. Take your time here.
Once the base is sealed, add a little mass to it by gathering slightly more glass from the bottom before flattening it for the final time. A slightly convex base that you flatten against the marver produces a flat, stable profile. Some artists prefer a slightly footed base — a subtle ring around the perimeter that lifts the body of the jar — which is formed by pressing only the outer rim of the base onto the marver while the center stays slightly domed.
A thick, flat base is your foundation — take extra time here and don't rush toward the walls. Base problems discovered after you've built the walls are almost impossible to fix without compromising the whole piece.
How do you build consistent walls on a glass jar?
With the base sealed and cooled to a stable working temperature, you move up the body of the tube — shortening and thickening where you want mass, or pulling and thinning where you want the walls to taper. The core discipline is rotation speed: slower rotations let you focus heat on a specific area, but too slow and you'll develop hot spots that sag or collapse. Keep the piece moving in a steady, deliberate rhythm and work in passes rather than sustained contact with the flame.
Check wall evenness regularly by holding the piece at arm's length and looking through the glass toward a light source. Thin spots appear as bright windows through the glass; thick areas are darker. An uneven wall doesn't just look wrong — it will create differential thermal expansion stress during annealing and cooling, making the piece more likely to crack post-kiln. The goal is a consistent 2.5–3mm wall thickness all the way around and all the way up the body.
As you work higher up the tube, keep the base end slightly elevated so gravity doesn't pull material down into the base you've already formed. Some artists use a mandrel or rod attached to the top of the tube for control at this stage. The walls don't need to be perfectly cylindrical — slight taper toward the opening can actually make the piece stronger structurally and more comfortable to handle — but they do need to be even.
How do you finish the opening of a glass jar?
The opening is where the jar meets its lid, which makes precision here directly functional rather than just aesthetic. Start by cutting or scoring the top of the tube at your intended opening height, then flame-polishing the raw edge to remove any sharp spots. Work the edge evenly in short passes — uneven heating at this stage is the most common cause of a wavy opening that won't accept a lid.
Insert a graphite mandrel that fits the interior diameter of your opening and heat the rim while rotating the mandrel inside the piece. The mandrel acts as a form, keeping the opening perfectly round while the heat allows you to coax it to shape. This is the same principle used in scientific glassblowing to produce precision-fit openings on reaction vessels, and it's the most reliable way to get a consistent diameter.
Once you're satisfied with the shape, measure the finished opening diameter with calipers and write it down immediately — you'll need that number when you form the lid. A finished jar opening typically runs 1–2mm larger in diameter than the lid's outer edge will be, accounting for the slight variation in fit and the fact that a borosilicate lid should rest on the opening, not be forced into it.
How do you make a matching lid for a glass jar?
Lid making is widely considered the most technically demanding part of jar construction, and with good reason. You're essentially making a free-form disc of a specific diameter — a shape that has no natural structural tendency to hold its form the way a tube or a bowl does. The glass wants to slump, pull in, or go asymmetric the moment you lose control of heat and rotation.
Start with a length of rod or a short piece of tubing roughly the same diameter as your jar opening. Heat the end of the rod until you have a molten gather, then press it onto the flat graphite marver to form a disc. Work the disc outward by alternating heat passes and marvering until you have a flat, even circle at approximately your target diameter. The disc doesn't need to be thick — 5–7mm is typical for a functional jar lid — but it does need to be consistent in thickness from edge to center.
The knob handle is formed while the disc is still soft. Touch a second rod to the center of the disc, heat the contact point until they bond, then pull the second rod upward while keeping the disc rotating. A short, solid knob of 10–15mm is easiest to grip and least likely to break in use. Some artists prefer a loop or ring handle instead, which is formed by pulling a loop from the center and bonding its tip back to the disc surface.
Test-fit the lid on the jar body while both are still warm enough to adjust — cold testing misses the small thermal expansion differences that can make a tight-fitting lid feel perfect at room temperature and too loose at working temperature, or vice versa. If the lid is too large, heat the rim and work it inward slightly. If it's too small, gently heat and stretch the rim outward while marvering. Most artists accept a fit that has 0.5–1mm of play — enough that the lid seats without gaps but lifts off easily without sticking.
How do you anneal borosilicate glass jars?
Proper annealing is more critical for jars than for almost any other boro form because of their mass. A jar stores significantly more thermal energy than a pipe, and that energy has to be released evenly or it creates internal stress that fractures the piece — sometimes instantly on opening the kiln, sometimes hours or days later as residual stress finds a weak point. The larger and heavier the jar, the longer the soak and the slower the ramp-down needs to be.
As a working baseline: soak at 560°C (1040°F) for a minimum of 30 minutes for a small jar (under 200g), 45 minutes for a medium jar, and 60 minutes or more for a large or thick-walled piece. From the soak temperature, ramp down no faster than 50–75°F per hour through the critical cooling range (560°C down to about 400°C). Below 400°C you can increase the ramp speed. Our full annealing guide covers schedule programming in detail, and if you're choosing a kiln, see our kiln buying guide for models with programmable controllers that make scheduled ramps simple.
Anneal the lid and body together if possible — they'll be used together and having matched thermal histories reduces stress in use. Place both pieces so they don't touch each other or the kiln walls. If the lid is significantly lighter than the body, it will reach temperature equilibrium faster, which is fine — just make sure both are in the kiln for the full soak period.
What are borosilicate glass jars used for?
The functional case for borosilicate jars is strong. The material doesn't leach chemicals, doesn't absorb odors, and won't interact with whatever is stored inside — properties that matter to anyone storing herbs, spices, teas, or other organic materials where glass's neutrality is part of the value proposition. A well-sealed boro jar with a fitted lid is also genuinely airtight, which extends the shelf life of aromatic materials far beyond what a loose-fitting lid provides.
As a market item, heady borosilicate jars sit at a favorable price point. The jar form is universally understood — everyone knows what a jar is and what it does — which lowers the barrier to purchase compared to more esoteric glass forms. Surface treatments like silver fuming, color overlays, and decorative inclusions command premium prices and allow your jar work to develop a recognizable signature style. Artists who want to build toward selling their work should read our guides to selling glass art and direct-sales channels for glass artists, which cover festivals, platforms, and commission pricing in detail.