My Glass Jars Have Tiny Air Bubbles Inside the Glass — How Do I Determine Whether They Are Cosmetic or Require Supplier Review Before Candle Use?

What those tiny bubbles in the glass usually are

Small bubbles sealed inside the wall of a glass jar are a normal feature of commercially made glass. Glassmakers often call the smallest ones seeds and larger ones blisters. They form when gas does not fully escape from the molten glass before it is shaped. A jar with a few fine seeds is very common, and in most cases the bubbles are purely cosmetic: they sit enclosed within solid glass and are not a crack, a chip or an opening.

That does not mean every bubble can be ignored. A bubble changes the glass locally, and three things decide whether it deserves attention: how big it is, where it sits, and whether it is closed inside the wall or open at a surface. A goods-in check for bubbles is really a way of sorting the ordinary cosmetic ones from the few that need a second look or a conversation with the supplier.

The 2 mm size and every category above are a suggested starting point for your own records, not a glass-industry grading and not a CSI specification. Formal inspection standards exist in the glass trade; for a candle business, a clear written rule that you apply consistently matters more than borrowing someone else's numbers.

How to inspect for bubbles quickly and consistently

Bubbles are easy to miss under ordinary room light and easy to over-count under a harsh lamp. Set up one inspection position and use it for every shipment so your counts are comparable.

Coloured, matte and frosted jars hide bubbles well, which cuts both ways. Customers will rarely notice a seed, but neither will you notice a blister near the rim. For opaque finishes, rely more on the cotton pad and on a close look at the rim and inner surface, where a bright torch shone into the jar often reveals what the outside conceals. Record that the check was limited by the finish, so nobody later assumes the glass was clear of defects when it simply could not be seen.

Worked example: 200 clear salsa jars with visible seeds

Suppose you receive 200 clear 200 ml salsa jars, bought as two packs of 100 at ₹2,950.00 each, so ₹5,900.00 in total and ₹29.50 per jar (₹2,950.00 ÷ 100). Clear glass shows every seed, so you inspect all 200 rather than a sample.

Your tally reads: 23 jars with a few fine enclosed seeds, 6 with enclosed blisters of about 2–3 mm in the wall, 2 with a blister in the rim, and 1 with an open pit on the inner surface. The illustrative decision works out like this.

Nine jars held back out of 200 is 4.5% of the shipment and ₹265.50 of glass. That is worth recording and reporting, but it is not a reason to reject the other 191 jars, most of which passed cleanly. Keep one jar from each category with its tally sheet so you can show the supplier exactly what you found.

When a conditional release makes sense

The six jars with enclosed blisters are the grey area. Fully enclosed blisters in the wall are often harmless, but a large one reduces the glass thickness locally, and you have no data on how that behaves through a full burn in your candle. A small supervised test settles it.

Pour three of the six as test candles using your standard formula and wick. Burn them fully alongside a candle in a bubble-free jar from the same shipment, stopping with about 1 cm of wax left as general practice for containers, and never leaving a test unattended. Extinguish any that smokes or flares, and inspect the glass around each blister after every cooling. If all three finish with the blister unchanged and no new lines in the glass, release the remaining three for production. If any shows a line, a crack or anything you cannot explain, remove all six and report it.

When to take it to the supplier

Raise it with the supplier when the pattern points to the glass run rather than the odd jar: bubbles concentrated in one carton, repeated bubbles in the rim or base corner, open blisters, or a sudden rise in seeds compared with your last shipment. For CSI jars, send the photographs and your tally on WhatsApp (+91 7397976926). CSI does not publish glass defect limits or tolerances, so the conversation starts from your evidence and your written acceptance rule.

Keep the tone factual. A clear tally with a photograph of each category gets a better response than a general complaint about bubbly glass. And apply the rule the same way whoever the supplier is, including us: a jar with a bubble in the rim stays out of your candles regardless of the invoice it arrived on.

Finally, any bubble that has a crack running from it, or any jar where you cannot decide whether a line is a crack, comes out of candle production without further testing. Heat is not a diagnostic tool for suspect glass.

Where this page fits

CSI already has guides that cover parts of this question. This page covers the goods-in check for the question above; these go deeper on the rest:

Three things to buy for this job

Frequently asked questions